#set document(title: "4.3 Projectile Motion", author: "OpenStax / XYZ Homework") #set page(width: 8.5in, height: auto, margin: 1in) #import "@preview/cetz:0.5.2" #set text(font: ("STIX Two Text", "Libertinus Serif", "New Computer Modern"), size: 10.5pt, lang: "en") #show math.equation: set text(font: ("STIX Two Math", "New Computer Modern Math")) #set par(justify: true, leading: 0.62em, spacing: 0.9em) #set enum(spacing: 1.1em) // room between list items so tall inline fractions don't collide #set list(spacing: 1.1em) #set table(stroke: 0.5pt + rgb("#c7ccd3")) #let BLUE = rgb("#183B6F") // brand navy — section bars + example/solution labels (white on navy 11.09:1) #let ORANGE = rgb("#A94509") // brand primary-700 — AA-safe deep orange for TEXT (5.93:1 on white; raw brand #F37021 is 2.94:1 and must never carry text) #let RED = rgb("#DC2626") // brand error-600 #let GREEN = rgb("#059669") // brand success-600 (decoration only; small green text uses green-text #007942) #show heading.where(level: 1): it => block(width: 100%, above: 0pt, below: 16pt, fill: gradient.linear(BLUE, rgb("#2C5AA0")), inset: (x: 14pt, y: 12pt), radius: 3pt, text(fill: white, weight: "bold", size: 19pt, it.body)) #show heading.where(level: 2): it => block(width: 100%, above: 18pt, below: 10pt, fill: BLUE, inset: (x: 10pt, y: 6pt), radius: 2pt, text(fill: white, weight: "bold", size: 12pt, it.body)) #show heading.where(level: 3): it => text(fill: ORANGE, weight: "bold", size: 12.5pt, it.body) #show heading.where(level: 4): it => text(fill: BLUE, weight: "bold", size: 10.5pt, it.body) #let examplebox(label, title, body) = block(width: 100%, breakable: true, fill: rgb("#EFF1F5"), stroke: 0.5pt + rgb("#CFDDF0"), radius: 4pt, inset: 10pt, above: 12pt, below: 12pt)[ #block(below: 6pt)[#box(fill: BLUE, inset: (x: 6pt, y: 2pt), radius: 2pt, text(fill: white, weight: "bold", size: 8.5pt, label)) #h(0.4em) #strong[#title]] #body] // rail = decorative left rule (raw brand token); labelcolor = AA-safe label text shade #let notebox(label, rail, labelcolor, tint, body) = block(width: 100%, breakable: true, fill: tint, stroke: (left: 3pt + rail), inset: (left: 10pt, rest: 8pt), radius: (right: 4pt), above: 11pt, below: 11pt)[ #text(fill: labelcolor, weight: "bold", size: 7.5pt, tracking: 0.5pt)[#upper(label)] #linebreak() #body] #let solutionbox(body) = block(above: 4pt, below: 8pt)[ #text(fill: BLUE, weight: "bold", size: 8.5pt)[Solution] #linebreak() #body] #let figph(msg) = block(width: 100%, height: 60pt, fill: rgb("#f6f7f9"), stroke: (paint: rgb("#c7ccd3"), dash: "dashed"), radius: 4pt, inset: 10pt)[ #align(center + horizon, text(fill: rgb("#889"), style: "italic", size: 9pt, msg))] // Standardize inlined figure sizes: measure the natural CeTZ canvas, then scale to a // consistent envelope (aspect-aware; see build_typst.py FIG_* constants). Unlike the // print preamble, dimensions are FLOORED: in an editor a user can trim a figure to a // degenerate 1-D shape (a bare line), and w/h or tw/w would then divide by zero. #let _STD_W = 3.5 #let _WIDE_W = 5.6 #let _MAX_H = 3.4 #let _ASPECT_WIDE = 2.2 #let _UPSCALE_MAX = 1.15 #let stdfig(body) = context { let m = measure(body) let w = calc.max(m.width / 1in, 0.01) let h = calc.max(m.height / 1in, 0.01) let tw = if w / h > _ASPECT_WIDE { _WIDE_W } else { _STD_W } let s = calc.min(tw / w, _MAX_H / h, _UPSCALE_MAX) align(center, box(scale(x: s * 100%, y: s * 100%, reflow: true, body))) } #show figure: set block(breakable: false) #set figure(gap: 8pt) #show figure.caption: set text(size: 8.5pt, fill: rgb("#555")) == 4.3#h(0.6em)Projectile Motion #strong[Projectile motion] is the motion of an object thrown or projected into the air, subject only to acceleration as a result of gravity. The applications of projectile motion in physics and engineering are numerous. Some examples include meteors as they enter Earth’s atmosphere, fireworks, and the motion of any ball in sports. Such objects are called #emph[projectiles] and their path is called a #strong[trajectory]. The motion of falling objects as discussed in Motion Along a Straight Line is a simple one-dimensional type of projectile motion in which there is no horizontal movement. In this section, we consider two-dimensional projectile motion, and our treatment neglects the effects of air resistance. The most important fact to remember here is that #emph[motions along perpendicular axes are independent] and thus can be analyzed separately. We discussed this fact in Displacement and Velocity Vectors, where we saw that vertical and horizontal motions are independent. The key to analyzing two-dimensional projectile motion is to break it into two motions: one along the horizontal axis and the other along the vertical. (This choice of axes is the most sensible because acceleration resulting from gravity is vertical; thus, there is no acceleration along the horizontal axis when air resistance is negligible.) As is customary, we call the horizontal axis the #emph[x]-axis and the vertical axis the #emph[y]-axis. It is not required that we use this choice of axes; it is simply convenient in the case of gravitational acceleration. In other cases we may choose a different set of axes. illustrates the notation for displacement, where we define #math.equation(block: false, alt: "s →")[$arrow(s)$] to be the total displacement, and #math.equation(block: false, alt: "x →")[$arrow(x)$] and #math.equation(block: false, alt: "y →")[$arrow(y)$] are its component vectors along the horizontal and vertical axes, respectively. The magnitudes of these vectors are #emph[s], #emph[x], and #emph[y]. #figure(figph[An illustration of a soccer player kicking a ball. The soccer player’s foot is at the origin of an x y coordinate system. The trajectory of the soccer ball and its location at 6 instants in time are shown. The trajectory is a parabola. The vector s is the displacement from the origin to the final position of the soccer ball. Vector s and its x and y components form a right triangle, with s as the hypotenuse and an angle phi between the x axis and s.], alt: "An illustration of a soccer player kicking a ball. The soccer player’s foot is at the origin of an x y coordinate system. The trajectory of the soccer ball and its location at 6 instants in time are shown. The trajectory is a parabola. The vector s is the displacement from the origin to the final position of the soccer ball. Vector s and its x and y components form a right triangle, with s as the hypotenuse and an angle phi between the x axis and s.", caption: [The total displacement #emph[s] of a soccer ball at a point along its path. The vector #math.equation(block: false, alt: "s →")[$arrow(s)$] has components #math.equation(block: false, alt: "x →")[$arrow(x)$] and #math.equation(block: false, alt: "y →")[$arrow(y)$] along the horizontal and vertical axes. Its magnitude is #emph[s] and it makes an angle #emph[Φ] with the horizontal.]) To describe #strong[projectile motion] completely, we must include velocity and acceleration, as well as displacement. We must find their components along the #emph[x-] and #emph[y]-axes. Let’s assume all forces except gravity (such as air resistance and friction, for example) are negligible. Defining the positive direction to be upward, the components of acceleration are then very simple: #math.equation(block: true, alt: "a sub y equals − g equals −9.8 m / s squared open parenthesis minus 32 ft / s squared close parenthesis .")[$a_(y) = "−" g = −9.8 #h(0.2em) "m" "/" "s"^(2) #h(0.5em) ( − 32 #h(0.2em) "ft" "/" "s"^(2) ) .$] Because gravity is vertical, #math.equation(block: false, alt: "a sub x equals 0 .")[$a_(x) = 0 .$] If #math.equation(block: false, alt: "a sub x equals 0 ,")[$a_(x) = 0 ,$] this means the initial velocity in the #emph[x] direction is equal to the final velocity in the #emph[x] direction, or #math.equation(block: false, alt: "v sub x equals v sub 0 x .")[$v_(x) = v_(0 x) .$] With these conditions on acceleration and velocity, we can write the kinematic through for motion in a uniform gravitational field, including the rest of the kinematic equations for a constant acceleration from Motion with Constant Acceleration. The kinematic equations for motion in a uniform gravitational field become kinematic equations with #math.equation(block: false, alt: "a sub y equals − g , a sub x equals 0 :")[$a_(y) = "−" g , #h(0.5em) a_(x) = 0 :$] Horizontal Motion #math.equation(block: true, alt: "v sub 0 x equals v sub x , x equals x sub 0 plus v sub x t")[$v_(0 x) = v_(x) , #h(0.2em) x = x_(0) + v_(x) t$] Vertical Motion #math.equation(block: true, alt: "y equals y sub 0 plus the fraction 1 over 2 open parenthesis v sub 0 y plus v sub y close parenthesis t")[$y = y_(0) + frac(1, 2) ( v_(0 y) + v_(y) ) t$] #math.equation(block: true, alt: "v sub y equals v sub 0 y minus g t")[$v_(y) = v_(0 y) − g t$] #math.equation(block: true, alt: "y equals y sub 0 plus v sub 0 y t minus the fraction 1 over 2 g t squared")[$y = y_(0) + v_(0 y) t − frac(1, 2) g t^(2)$] #math.equation(block: true, alt: "v y 2 equals v 0 y 2 minus 2 g open parenthesis y minus y sub 0 close parenthesis")[$v_(y)^(2) = v_(0 y)^(2) − 2 g ( y − y_(0) )$] Using this set of equations, we can analyze projectile motion, keeping in mind some important points. #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #emph[Projectile Motion] + Resolve the motion into horizontal and vertical components along the #emph[x]- and #emph[y]-axes. The magnitudes of the components of displacement #math.equation(block: false, alt: "s →")[$arrow(s)$] along these axes are #emph[x] and #emph[y.] The magnitudes of the components of velocity #math.equation(block: false, alt: "v →")[$arrow(v)$] are #math.equation(block: false, alt: "v sub x equals v cos θ and v sub y equals v sin θ ,")[$v_(x) = v "cos" #h(0.1em) θ #h(0.2em) "and" #h(0.2em) v_(y) = v "sin" #h(0.1em) θ ,$] where #emph[v] is the magnitude of the velocity and #emph[θ] is its direction relative to the horizontal, as shown. + Treat the motion as two independent one-dimensional motions: one horizontal and the other vertical. Use the kinematic equations for horizontal and vertical motion presented earlier. + Solve for the unknowns in the two separate motions: one horizontal and one vertical. Note that the only common variable between the motions is time #emph[t]. The problem-solving procedures here are the same as those for one-dimensional kinematics and are illustrated in the following solved examples. + Recombine quantities in the horizontal and vertical directions to find the total displacement #math.equation(block: false, alt: "s →")[$arrow(s)$] and velocity #math.equation(block: false, alt: "v → .")[$arrow(v) .$] Solve for the magnitude and direction of the displacement and velocity using #linebreak() #math.equation(block: true, alt: "s equals the square root of x squared plus y squared , Φ equals tan to the power −1 open parenthesis y / x close parenthesis , v equals the square root of v x 2 plus v y 2 ,")[$s = sqrt(x^(2) + y^(2)) , #h(0.5em) Φ = "tan"^(−1) ( y "/" x ) , #h(0.5em) v = sqrt(v_(x)^(2) + v_(y)^(2)) ,$] #linebreak() where #emph[Φ] is the direction of the displacement #math.equation(block: false, alt: "s → .")[$arrow(s) .$] ] #figure(figph[Figure a shows the locations and velocities of a projectile on an x y coordinate system at 10 instants in time. When the projectile is at the origin, it has a velocity v sub 0 y which makes an angle theta sub 0 with the horizontal. The velocity is shown as a dark blue arrow, and its x and y components are shown as light blue arrow. The projectile’s position follows a downward-opening parabola, moving up to a maximum height, then back to y = 0, and continuing below the x axis The velocity, V, at each time makes an angle theta which changes in time, and has x component V sub x and y component v sub y. The x component of the velocity V sub x is the same at all times. The y component v sub y points up but gets smaller, until the projectile reaches the maximum height, where the velocity is horizontal and has no y component. After the maximum height, the velocity has a y component pointing down and growing larger. As the projectile reaches the same elevation on the way down as it had on the way up, its velocity is below the horizontal by the same angle theta as it was above the horizontal on the way up. In particular, when it comes back to y = 0 on the way down, the angle between the vector v and the horizontal is minus s=theta sub zero and the y component of the velocity is minus v sub 0 y. The last position shown is below the x axis, and the y component of the velocity is larger than it was initially. The graph clearly shows that the horizontal distances travelled in each of the time intervals are equal, while the vertical distances decrease on the way up and increase on the way down. Figure b shows the horizontal component, constant velocity. The horizontal positions and x components of the velocity of the projectile are shown along a horizontal line. The positions are evenly spaced, and the x components of the velocities are all the same, and point to the right. Figure c shows the vertical component, constant acceleration. The vertical positions and y components of the velocity of the projectile are shown along a vertical line. The positions are get closer together on the way up, then further apart on the way down. The y components of the velocities initially point up, decreasing in magnitude until there is no y component to the velocity at the maximum height. After the maximum height, the y components of the velocities point down and increase in magnitude. Figure d shows that putting the horizontal and vertical components of figures b and c together gives the total velocity at a point. The velocity V has an x component of V sub x, has y component of V sub y, and makes an angle of theta with the horizontal. In the example shown, the velocity has a downward y component.], alt: "Figure a shows the locations and velocities of a projectile on an x y coordinate system at 10 instants in time. When the projectile is at the origin, it has a velocity v sub 0 y which makes an angle theta sub 0 with the horizontal. The velocity is shown as a dark blue arrow, and its x and y components are shown as light blue arrow. The projectile’s position follows a downward-opening parabola, moving up to a maximum height, then back to y = 0, and continuing below the x axis The velocity, V, at each time makes an angle theta which changes in time, and has x component V sub x and y component v sub y. The x component of the velocity V sub x is the same at all times. The y component v sub y points up but gets smaller, until the projectile reaches the maximum height, where the velocity is horizontal and has no y component. After the maximum height, the velocity has a y component pointing down and growing larger. As the projectile reaches the same elevation on the way down as it had on the way up, its velocity is below the horizontal by the same angle theta as it was above the horizontal on the way up. In particular, when it comes back to y = 0 on the way down, the angle between the vector v and the horizontal is minus s=theta sub zero and the y component of the velocity is minus v sub 0 y. The last position shown is below the x axis, and the y component of the velocity is larger than it was initially. The graph clearly shows that the horizontal distances travelled in each of the time intervals are equal, while the vertical distances decrease on the way up and increase on the way down. Figure b shows the horizontal component, constant velocity. The horizontal positions and x components of the velocity of the projectile are shown along a horizontal line. The positions are evenly spaced, and the x components of the velocities are all the same, and point to the right. Figure c shows the vertical component, constant acceleration. The vertical positions and y components of the velocity of the projectile are shown along a vertical line. The positions are get closer together on the way up, then further apart on the way down. The y components of the velocities initially point up, decreasing in magnitude until there is no y component to the velocity at the maximum height. After the maximum height, the y components of the velocities point down and increase in magnitude. Figure d shows that putting the horizontal and vertical components of figures b and c together gives the total velocity at a point. The velocity V has an x component of V sub x, has y component of V sub y, and makes an angle of theta with the horizontal. In the example shown, the velocity has a downward y component.", caption: [(a) We analyze two-dimensional projectile motion by breaking it into two independent one-dimensional motions along the vertical and horizontal axes. (b) The horizontal motion is simple, because #math.equation(block: false, alt: "a sub x equals 0")[$a_(x) = 0$] and #math.equation(block: false, alt: "v sub x")[$v_(x)$] is a constant. (c) The velocity in the vertical direction begins to decrease as the object rises. At its highest point, the vertical velocity is zero. As the object falls toward Earth again, the vertical velocity increases again in magnitude but points in the opposite direction to the initial vertical velocity. (d) The #emph[x] and #emph[y] motions are recombined to give the total velocity at any given point on the trajectory.]) #examplebox("Example 1")[A Fireworks Projectile Explodes High and Away][ During a fireworks display, a shell is shot into the air with an initial speed of 70.0 m/s at an angle of #math.equation(block: false, alt: "75.0 °")[$75.0 "°"$] above the horizontal, as illustrated in . The fuse is timed to ignite the shell just as it reaches its highest point above the ground. (a) Calculate the height at which the shell explodes. (b) How much time passes between the launch of the shell and the explosion? (c) What is the horizontal displacement of the shell when it explodes? (d) What is the total displacement from the point of launch to the highest point? #figure(figph[The trajectory of a fireworks shell from its launch to its highest point is shown as the left half of a downward-opening parabola in a graph of y as a function of x. The maximum height is h = 233 meters and its x displacement at that time is x = 125 meters. The initial velocity vector v sub 0 is up and to the right, tangent to the trajectory curve, and makes an angle of theta sub 0 equal to 75 degrees.], alt: "The trajectory of a fireworks shell from its launch to its highest point is shown as the left half of a downward-opening parabola in a graph of y as a function of x. The maximum height is h = 233 meters and its x displacement at that time is x = 125 meters. The initial velocity vector v sub 0 is up and to the right, tangent to the trajectory curve, and makes an angle of theta sub 0 equal to 75 degrees.", caption: [The trajectory of a fireworks shell. The fuse is set to explode the shell at the highest point in its trajectory, which is found to be at a height of 233 m and 125 m away horizontally.]) Strategy The motion can be broken into horizontal and vertical motions in which #math.equation(block: false, alt: "a sub x equals 0")[$a_(x) = 0$] and #math.equation(block: false, alt: "a sub y equals − g .")[$a_(y) = "−" g .$] We can then define #math.equation(block: false, alt: "x sub 0")[$x_(0)$] and #math.equation(block: false, alt: "y sub 0")[$y_(0)$] to be zero and solve for the desired quantities. Solution (a) By “height” we mean the altitude or vertical position #emph[y] above the starting point. The highest point in any trajectory, called the #emph[apex], is reached when #math.equation(block: false, alt: "v sub y equals 0 .")[$v_(y) = 0 .$] Since we know the initial and final velocities, as well as the initial position, we use the following equation to find #emph[y]: #math.equation(block: true, alt: "v y 2 equals v 0 y 2 minus 2 g open parenthesis y minus y sub 0 close parenthesis .")[$v_(y)^(2) = v_(0 y)^(2) − 2 g ( y − y_(0) ) .$] Because #math.equation(block: false, alt: "y sub 0")[$y_(0)$] and #math.equation(block: false, alt: "v sub y")[$v_(y)$] are both zero, the equation simplifies to #math.equation(block: true, alt: "0 equals v 0 y 2 minus 2 g y .")[$"0" = v_(0 y)^(2) − 2 g y .$] Solving for #emph[y] gives #math.equation(block: true, alt: "y equals the fraction v 0 y 2 over 2 g .")[$y = frac(v_(0 y)^(2), 2 g) .$] Now we must find #math.equation(block: false, alt: "v sub 0 y ,")[$v_(0 y) ,$] the component of the initial velocity in the #emph[y] direction. It is given by #math.equation(block: false, alt: "v sub 0 y equals v sub 0 sin θ sub 0 ,")[$v_(0 y) = v_(0) "sin" θ_(0) ,$] where #math.equation(block: false, alt: "v sub 0")[$v_(0)$] is the initial velocity of 70.0 m/s and #math.equation(block: false, alt: "θ sub 0 equals 75 °")[$θ_(0) = 75 "°"$] is the initial angle. Thus, #math.equation(block: true, alt: "v sub 0 y equals v sub 0 sin θ equals open parenthesis 70.0 m / s close parenthesis sin 75 ° equals 67.6 m / s")[$v_(0 y) = v_(0) "sin" #h(0.1em) θ = ( 70.0 #h(0.2em) "m" "/" "s" ) "sin" #h(0.2em) 75 "°" = 67.6 #h(0.2em) "m" "/" "s"$] and #emph[y] is #math.equation(block: true, alt: "y equals the fraction open parenthesis 67.6 m / s close parenthesis squared over 2 open parenthesis 9.80 m / s squared close parenthesis .")[$y = frac(attach(( 67.6 #h(0.2em) "m" "/" "s" ), t: 2), 2 ( 9.80 #h(0.2em) "m" "/" "s"^(2) )) .$] Thus, we have #math.equation(block: true, alt: "y equals 233 m .")[$y = 233 #h(0.2em) "m" "."$] Note that because up is positive, the initial vertical velocity is positive, as is the maximum height, but the acceleration resulting from gravity is negative. Note also that the maximum height depends only on the vertical component of the initial velocity, so that any projectile with a 67.6-m/s initial vertical component of velocity reaches a maximum height of 233 m (neglecting air resistance). The numbers in this example are reasonable for large fireworks displays, the shells of which do reach such heights before exploding. In practice, air resistance is not completely negligible, so the initial velocity would have to be somewhat larger than that given to reach the same height. (b) As in many physics problems, there is more than one way to solve for the time the projectile reaches its highest point. In this case, the easiest method is to use #math.equation(block: false, alt: "v sub y equals v sub 0 y minus g t .")[$v_(y) = v_(0 y) − g t .$] Because #math.equation(block: false, alt: "v sub y equals 0")[$v_(y) = 0$] at the apex, this equation reduces to simply #math.equation(block: true, alt: "0 equals v sub 0 y minus g t")[$0 = v_(0 y) − g t$] or #math.equation(block: true, alt: "t equals the fraction v sub 0 y over g equals the fraction 67.6 m / s over 9.80 m / s squared equals 6.90 s .")[$t = frac(v_(0 y), g) = frac(67.6 #h(0.2em) "m" "/" "s", 9.80 #h(0.2em) "m" "/" "s"^(2)) = 6.90 "s" "."$] This time is also reasonable for large fireworks. If you are able to see the launch of fireworks, notice that several seconds pass before the shell explodes. Another way of finding the time is by using#math.equation(block: false, alt: "y = y sub 0 plus the fraction 1 over 2 open parenthesis v sub 0 y plus v sub y close parenthesis t .")[$y #h(0.2em) "=" #h(0.2em) y_(0) + frac(1, 2) ( v_(0 y) + v_(y) ) t .$] This is left for you as an exercise to complete. (c) Because air resistance is negligible, #math.equation(block: false, alt: "a sub x equals 0")[$a_(x) = 0$] and the horizontal velocity is constant, as discussed earlier. The horizontal displacement is the horizontal velocity multiplied by time as given by #math.equation(block: false, alt: "x equals x sub 0 plus v sub x t ,")[$x = x_(0) + v_(x) t ,$] where #math.equation(block: false, alt: "x sub 0")[$x_(0)$] is equal to zero. Thus, #math.equation(block: true, alt: "x equals v sub x t ,")[$x = v_(x) t ,$] where #math.equation(block: false, alt: "v sub x")[$v_(x)$] is the #emph[x]-component of the velocity, which is given by #math.equation(block: true, alt: "v sub x equals v sub 0 cos θ equals open parenthesis 70.0 m / s close parenthesis cos 75 ° equals 18.1 m / s .")[$v_(x) = v_(0) "cos" #h(0.1em) θ = ( 70.0 #h(0.2em) "m" "/" "s" ) "cos" 75 "°" = 18.1 #h(0.2em) "m" "/" "s" .$] Time #emph[t] for both motions is the same, so #emph[x] is #math.equation(block: true, alt: "x equals open parenthesis 18.1 m / s close parenthesis 6.90 s equals 125 m .")[$x = ( 18.1 #h(0.2em) "m" "/" "s" ) 6.90 #h(0.2em) "s" = 125 #h(0.2em) "m" "."$] Horizontal motion is a constant velocity in the absence of air resistance. The horizontal displacement found here could be useful in keeping the fireworks fragments from falling on spectators. When the shell explodes, air resistance has a major effect, and many fragments land directly below. (d) The horizontal and vertical components of the displacement were just calculated, so all that is needed here is to find the magnitude and direction of the displacement at the highest point: #math.equation(block: true, alt: "s → equals 125 i ^ plus 233 j ^")[$arrow(s) = 125 hat(i) + 233 hat(j)$] #math.equation(block: true, alt: "| s → | equals the square root of 125 squared plus 233 squared equals 264 m")[$| arrow(s) | = sqrt(125^(2) + 233^(2)) = 264 #h(0.2em) "m"$] #math.equation(block: true, alt: "Φ equals tan to the power −1 open parenthesis the fraction 233 over 125 close parenthesis equals 61.8 ° .")[$Φ = "tan"^(−1) ( frac(233, 125) ) = 61.8 "°" .$] Note that the angle for the displacement vector is less than the initial angle of launch. To see why this is, review , which shows the curvature of the trajectory toward the ground level. ] When solving (a), the expression we found for #emph[y] is valid for any projectile motion when air resistance is negligible. Call the maximum height #emph[y] = #emph[h]. Then, #math.equation(block: true, alt: "h equals the fraction v 0 y 2 over 2 g .")[$h = frac(v_(0 y)^(2), 2 g) .$] This equation defines the #emph[maximum height of a projectile above its launch position] and it depends only on the vertical component of the initial velocity. #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ A rock is thrown horizontally off a cliff #math.equation(block: false, alt: "100.0 m")[$100.0 #h(0.2em) "m"$] high with a velocity of 15.0 m/s. (a) Define the origin of the coordinate system. (b) Which equation describes the horizontal motion? (c) Which equations describe the vertical motion? (d) What is the rock’s velocity at the point of impact? #solutionbox[ (a) Choose the top of the cliff where the rock is thrown from the origin of the coordinate system. Although it is arbitrary, we typically choose time #emph[t] = 0 to correspond to the origin. (b) The equation that describes the horizontal motion is #math.equation(block: false, alt: "x equals x sub 0 plus v sub x t .")[$x = x_(0) + v_(x) t .$] With #math.equation(block: false, alt: "x sub 0 equals 0 ,")[$x_(0) = 0 ,$] this equation becomes #math.equation(block: false, alt: "x equals v sub x t .")[$x = v_(x) t .$] (c) through and describe the vertical motion, but since #math.equation(block: false, alt: "y sub 0 equals 0 and v sub 0 y equals 0 ,")[$y_(0) = 0 #h(0.2em) "and" #h(0.2em) v_(0 y) = 0 ,$] these equations simplify greatly to become #math.equation(block: false, alt: "y equals the fraction 1 over 2 open parenthesis v sub 0 y plus v sub y close parenthesis t equals the fraction 1 over 2 v sub y t ,")[$y = frac(1, 2) ( v_(0 y) + v_(y) ) t = frac(1, 2) v_(y) t , #h(0.5em)$]#math.equation(block: false, alt: "v sub y equals − g t ,")[$v_(y) = "−" g t , #h(0.5em)$]#math.equation(block: false, alt: "y equals minus the fraction 1 over 2 g t squared ,")[$y = − frac(1, 2) g t^(2) , #h(0.5em)$] and #math.equation(block: false, alt: "v y 2 equals −2 g y .")[$v_(y)^(2) = −2 g y .$] (d) We use the kinematic equations to find the #emph[x] and #emph[y] components of the velocity at the point of impact. Using #math.equation(block: false, alt: "v y 2 equals −2 g y")[$v_(y)^(2) = −2 g y$] and noting the point of impact is −100.0 m, we find the #emph[y] component of the velocity at impact is #math.equation(block: false, alt: "v sub y equals 44.3 m / s .")[$v_(y) = 44.3 #h(0.2em) "m" "/" "s" .$] We are given the #emph[x] component, #math.equation(block: false, alt: "v sub x equals 15.0 m / s ,")[$v_(x) = 15.0 #h(0.2em) "m" "/" "s" ,$] so we can calculate the total velocity at impact: #emph[v] = 46.8 m/s and #math.equation(block: false, alt: "θ equals 71.3 °")[$θ = 71.3 "°"$] below the horizontal. ] ] #examplebox("Example 2")[Calculating Projectile Motion: Tennis Player][ A tennis player wins a match at Arthur Ashe stadium and hits a ball into the stands at 30 m/s and at an angle #math.equation(block: false, alt: "45 °")[$45 "°"$] above the horizontal . On its way down, the ball is caught by a spectator 10 m above the point where the ball was hit. (a) Calculate the time it takes the tennis ball to reach the spectator. (b) What are the magnitude and direction of the ball’s velocity at impact? #figure(figph[An illustration of a tennis ball launched into the stands. The player is to the left of the stands and hits the ball up and to the right at an angle of theta equal to 45 degrees and velocity of v sub 0 equal to 30 meters per second. The ball reaches a spectator who is seated 10 meters above the initial height of the ball.], alt: "An illustration of a tennis ball launched into the stands. The player is to the left of the stands and hits the ball up and to the right at an angle of theta equal to 45 degrees and velocity of v sub 0 equal to 30 meters per second. The ball reaches a spectator who is seated 10 meters above the initial height of the ball.", caption: [The trajectory of a tennis ball hit into the stands.]) Strategy Again, resolving this two-dimensional motion into two independent one-dimensional motions allows us to solve for the desired quantities. The time a projectile is in the air is governed by its vertical motion alone. Thus, we solve for #emph[t] first. While the ball is rising and falling vertically, the horizontal motion continues at a constant velocity. This example asks for the final velocity. Thus, we recombine the vertical and horizontal results to obtain #math.equation(block: false, alt: "v →")[$arrow(v)$] at final time #emph[t], determined in the first part of the example. Solution (a) While the ball is in the air, it rises and then falls to a final position 10.0 m higher than its starting altitude. We can find the time for this by using : #math.equation(block: true, alt: "y = y sub 0 + v sub 0 y t minus the fraction 1 over 2 g t squared .")[$y #h(0.2em) "=" #h(0.2em) y_(0) #h(0.2em) "+" #h(0.2em) v_(0 y) t − frac(1, 2) g t^(2) .$] If we take the initial position #math.equation(block: false, alt: "y sub 0")[$y_(0)$] to be zero, then the final position is #emph[y] = 10 m. The initial vertical velocity is the vertical component of the initial velocity: #math.equation(block: true, alt: "v sub 0 y equals v sub 0 sin θ sub 0 equals open parenthesis 30.0 m / s close parenthesis sin 45 ° equals 21.2 m / s .")[$v_(0 y) = v_(0) "sin" #h(0.1em) θ_(0) = ( 30.0 #h(0.2em) "m" "/" "s" ) "sin" #h(0.2em) 45 "°" = 21.2 #h(0.2em) "m" "/" "s" .$] Substituting into for #emph[y] gives us #math.equation(block: true, alt: "10.0 m equals open parenthesis 21.2 m/s close parenthesis t minus open parenthesis 4.90 m/s squared close parenthesis t squared .")[$10.0 #h(0.2em) "m" = ( 21.2 #h(0.2em) "m/s" ) t − ( 4.90 #h(0.2em) "m/s"^("2") ) t^(2) .$] Rearranging terms gives a quadratic equation in #emph[t]: #math.equation(block: true, alt: "open parenthesis 4.90 m/s squared close parenthesis t squared minus open parenthesis 21.2 m/s close parenthesis t plus 10.0 m equals 0 .")[$( 4.90 #h(0.2em) "m/s"^("2") ) t^(2) − ( 21.2 #h(0.2em) "m/s" ) t + 10.0 #h(0.2em) "m" = 0 .$] Use of the quadratic formula yields #emph[t] = 3.79 s and #emph[t] = 0.54 s. Since the ball is at a height of 10 m at two times during its trajectory—once on the way up and once on the way down—we take the longer solution for the time it takes the ball to reach the spectator: #math.equation(block: true, alt: "t equals 3.79 s .")[$t = 3.79 #h(0.2em) "s" "."$] The time for projectile motion is determined completely by the vertical motion. Thus, any projectile that has an initial vertical velocity of 21.2 m/s and lands 10.0 m above its starting altitude spends 3.79 s in the air. (b) We can find the final horizontal and vertical velocities #math.equation(block: false, alt: "v sub x")[$v_(x)$] and #math.equation(block: false, alt: "v sub y")[$v_(y)$] with the use of the result from (a). Then, we can combine them to find the magnitude of the total velocity vector #math.equation(block: false, alt: "v →")[$arrow(v)$] and the angle #math.equation(block: false, alt: "θ")[$θ$] it makes with the horizontal. Since #math.equation(block: false, alt: "v sub x")[$v_(x)$] is constant, we can solve for it at any horizontal location. We choose the starting point because we know both the initial velocity and the initial angle. Therefore, #math.equation(block: true, alt: "v sub x equals v sub 0 cos θ sub 0 equals open parenthesis 30 m / s close parenthesis cos 45 ° equals 21.2 m / s .")[$v_(x) = v_(0) "cos" θ_(0) = ( 30 #h(0.2em) "m" "/" "s" ) "cos" #h(0.2em) 45 "°" = 21.2 #h(0.2em) "m" "/" "s" .$] The final vertical velocity is given by : #math.equation(block: true, alt: "v sub y equals v sub 0 y minus g t .")[$v_(y) = v_(0 y) − g t .$] Since #math.equation(block: false, alt: "v sub 0 y")[$v_(0 y)$] was found in part (a) to be 21.2 m/s, we have #math.equation(block: true, alt: "v sub y equals 21.2 m / s minus 9.8 m / s squared open parenthesis 3.79 s close parenthesis equals −15.9 m / s .")[$v_(y) = 21.2 #h(0.2em) "m" "/" "s" − 9.8 #h(0.2em) "m" "/" "s"^(2) ( 3.79 #h(0.2em) "s" ) = −15.9 #h(0.2em) "m" "/" "s" .$] The magnitude of the final velocity #math.equation(block: false, alt: "v →")[$arrow(v)$] is #math.equation(block: true, alt: "v equals the square root of v x 2 plus v y 2 equals the square root of open parenthesis 21.2 m / s close parenthesis squared plus open parenthesis − 15 .9 m / s close parenthesis squared equals 26.5 m / s .")[$v = sqrt(v_(x)^(2) + v_(y)^(2)) = sqrt(attach(( 21.2 #h(0.2em) "m" "/" "s" ), t: 2) + attach(( "−" #h(0.2em) "15" .9 #h(0.2em) "m" "/" "s" ), t: 2)) = 26.5 #h(0.2em) "m" "/" "s" .$] The direction #math.equation(block: false, alt: "θ sub v")[$θ_(v)$] is found using the inverse tangent: #math.equation(block: true, alt: "θ sub v equals tan to the power −1 open parenthesis the fraction v sub y over v sub x close parenthesis equals tan to the power −1 open parenthesis the fraction −15.9 over 21.2 close parenthesis equals 36.9 ° below the horizon.")[$θ_(v) = "tan"^(−1) ( frac(v_(y), v_(x)) ) = "tan"^(−1) ( frac(−15.9, 21.2) ) = 36.9 "°" "below" "the" "horizon".$] Significance (a) As mentioned earlier, the time for projectile motion is determined completely by the vertical motion. Thus, any projectile that has an initial vertical velocity of 21.2 m/s and lands 10.0 m above its starting altitude spends 3.79 s in the air. (b) The negative angle means the velocity is #math.equation(block: false, alt: "36.9 °")[$36.9 "°"$] below the horizontal at the point of impact. This result is consistent with the fact that the ball is impacting at a point on the other side of the apex of the trajectory and therefore has a negative #emph[y] component of the velocity. The magnitude of the velocity is less than the magnitude of the initial velocity we expect since it is impacting 10.0 m above the launch elevation. ] === Time of Flight, Trajectory, and Range Of interest are the time of flight, trajectory, and range for a projectile launched on a flat horizontal surface and impacting on the same surface. In this case, kinematic equations give useful expressions for these quantities, which are derived in the following sections. ==== Time of flight We can solve for the time of flight of a projectile that is both launched and impacts on a flat horizontal surface by performing some manipulations of the kinematic equations. We note the position and displacement in #emph[y] must be zero at launch and at impact on an even surface. Thus, we set the displacement in #emph[y] equal to zero and find #math.equation(block: true, alt: "y minus y sub 0 equals v sub 0 y t minus the fraction 1 over 2 g t squared equals open parenthesis v sub 0 sin θ sub 0 close parenthesis t minus the fraction 1 over 2 g t squared equals 0 .")[$y − y_(0) = v_(0 y) t − frac(1, 2) g t^(2) = ( v_(0) "sin" θ_(0) ) t − frac(1, 2) g t^(2) = 0 .$] Factoring, we have #math.equation(block: true, alt: "t open parenthesis v sub 0 sin θ sub 0 minus the fraction g t over 2 close parenthesis equals 0 .")[$t ( v_(0) "sin" θ_(0) − frac(g t, 2) ) = 0 .$] Solving for #emph[t] gives us #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #math.equation(block: true, alt: "T sub tof equals the fraction 2 open parenthesis v sub 0 sin θ sub 0 close parenthesis over g .")[$T_("tof") = frac(2 ( v_(0) "sin" θ_(0) ), g) .$] ] This is the #strong[time of flight] for a projectile both launched and impacting on a flat horizontal surface. does not apply when the projectile lands at a different elevation than it was launched, as we saw in of the tennis player hitting the ball into the stands. The other solution, #emph[t] = 0, corresponds to the time at launch. The time of flight is linearly proportional to the initial velocity in the #emph[y] direction and inversely proportional to #emph[g]. Thus, on the Moon, where gravity is one-sixth that of Earth, a projectile launched with the same velocity as on Earth would be airborne six times as long. ==== Trajectory The #strong[trajectory] of a projectile can be found by eliminating the time variable #emph[t] from the kinematic equations for arbitrary #emph[t] and solving for #emph[y] (#emph[x]). We take #math.equation(block: false, alt: "x sub 0 equals y sub 0 equals 0")[$x_(0) = y_(0) = 0$] so the projectile is launched from the origin. The kinematic equation for #emph[x] gives #math.equation(block: true, alt: "x equals v sub 0 x t ⇒ t equals the fraction x over v sub 0 x equals the fraction x over v sub 0 cos θ sub 0 .")[$x = v_(0 x) t ⇒ t = frac(x, v_(0 x)) = frac(x, v_(0) "cos" θ_(0)) .$] Substituting the expression for #emph[t] into the equation for the position #math.equation(block: false, alt: "y equals open parenthesis v sub 0 sin θ sub 0 close parenthesis t minus the fraction 1 over 2 g t squared")[$y = ( v_(0) "sin" θ_(0) ) t − frac(1, 2) g t^(2)$] gives #math.equation(block: true, alt: "y equals open parenthesis v sub 0 sin θ sub 0 close parenthesis open parenthesis the fraction x over v sub 0 cos θ sub 0 close parenthesis minus the fraction 1 over 2 g open parenthesis the fraction x over v sub 0 cos θ sub 0 close parenthesis squared .")[$y = ( v_(0) "sin" #h(0.1em) θ_(0) ) ( frac(x, v_(0) "cos" #h(0.1em) θ_(0)) ) − frac(1, 2) g attach(( frac(x, v_(0) "cos" #h(0.1em) θ_(0)) ), t: 2) .$] Rearranging terms, we have #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #math.equation(block: true, alt: "y equals open parenthesis tan θ sub 0 close parenthesis x minus [ the fraction g over 2 open parenthesis v sub 0 cos θ sub 0 close parenthesis squared ] x squared .")[$y = ( "tan" #h(0.1em) θ_(0) ) x − [ frac(g, 2 attach(( v_(0) "cos" #h(0.1em) θ_(0) ), t: 2)) ] x^(2) .$] ] This trajectory equation is of the form #math.equation(block: false, alt: "y equals a x plus b x squared ,")[$y = a x + b x^(2) ,$] which is an equation of a parabola with coefficients #math.equation(block: true, alt: "a equals tan θ sub 0 , b equals minus the fraction g over 2 open parenthesis v sub 0 cos θ sub 0 close parenthesis squared .")[$a = "tan" #h(0.1em) θ_(0) , #h(0.5em) b = − frac(g, 2 attach(( v_(0) "cos" #h(0.1em) θ_(0) ), t: 2)) .$] ==== Range From the trajectory equation we can also find the #strong[range], or the horizontal distance traveled by the projectile. Factoring , we have #math.equation(block: true, alt: "y equals x [ tan θ sub 0 minus the fraction g over 2 open parenthesis v sub 0 cos θ sub 0 close parenthesis squared x ] .")[$y = x [ "tan" #h(0.1em) θ_(0) − frac(g, 2 attach(( v_(0) "cos" #h(0.1em) θ_(0) ), t: 2)) x ] .$] The position #emph[y] is zero for both the launch point and the impact point, since we are again considering only a flat horizontal surface. Setting #emph[y] = 0 in this equation gives solutions #emph[x] = 0, corresponding to the launch point, and #math.equation(block: true, alt: "x equals the fraction 2 v 0 2 sin θ sub 0 cos θ sub 0 over g ,")[$x = frac(2 v_(0)^(2) "sin" #h(0.1em) θ_(0) "cos" #h(0.1em) θ_(0), g) ,$] corresponding to the impact point. Using the trigonometric identity #math.equation(block: false, alt: "2 sin θ cos θ equals sin 2 θ")[$2 "sin" #h(0.1em) θ "cos" #h(0.1em) θ = "sin" 2 θ$] and setting #emph[x] = #emph[R] for range, we find #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #math.equation(block: true, alt: "R equals the fraction v 0 2 sin 2 θ sub 0 over g .")[$R = frac(v_(0)^(2) "sin" 2 θ_(0), g) .$] ] Note particularly that is valid only for launch and impact on a horizontal surface. We see the range is directly proportional to the square of the initial speed #math.equation(block: false, alt: "v sub 0")[$v_(0)$] and #math.equation(block: false, alt: "sin 2 θ sub 0")[$"sin" 2 θ_(0)$], and it is inversely proportional to the acceleration of gravity. Thus, on the Moon, the range would be six times greater than on Earth for the same initial velocity. Furthermore, we see from the factor #math.equation(block: false, alt: "sin 2 θ sub 0")[$"sin" 2 θ_(0)$] that the range is maximum at #math.equation(block: false, alt: "45 ° .")[$45 "°" .$] These results are shown. In (a) we see that the greater the initial velocity, the greater the range. In (b), we see that the range is maximum at #math.equation(block: false, alt: "45 ° .")[$45 "°" .$] This is true only for conditions neglecting air resistance. If air resistance is considered, the maximum angle is somewhat smaller. It is interesting that the same range is found for two initial launch angles that sum to #math.equation(block: false, alt: "90 ° .")[$90 "°" .$] The projectile launched with the smaller angle has a lower apex than the higher angle, but they both have the same range. Trajectories y(x) = x·tanθ − gx²/(2v₀²cos²θ) for the 70.0-m/s fireworks shell of Example 4.7 launched at θ (solid) and at the complementary angle 90° − θ (dashed). At the example's θ = 75° the shell climbs to the 233-m apex the text computes, and both curves come back to the ground together at R = v₀²sin2θ/g = 250 m. Drag θ and the pair always shares a range — the shallow shot is a flat liner, the steep one a lob — while the range itself is largest, 500 m, where the two curves merge at θ = 45°. Each curve is drawn past its landing point, so the plunge below the axis is the parabola continuing underground, not part of the flight.{"functions":\[{"color":"\#1f4e79","domain":\[0,500\],"expression":"tan(ang\*pi/180)\*x - 4.9\*x^2/(4900\*cos(ang\*pi/180)^2)","expression2":"","id":"traj","inequality":"lt","lineDash":\[\],"lineWidth":2.5,"mode":"cartesian","tMax":10,"tMin":-10,"variable":"x","visible":true},{"color":"\#b8442a","domain":\[0,500\],"expression":"tan((90-ang)\*pi/180)\*x - 4.9\*x^2/(4900\*cos((90-ang)\*pi/180)^2)","expression2":"","id":"twin","inequality":"lt","lineDash":\[6,4\],"lineWidth":2,"mode":"cartesian","tMax":10,"tMin":-10,"variable":"x","visible":true}\],"grid":{"axisColor":"\#333333","gridColor":"\#cccccc","majorSpacing":50,"minorGridColor":"\#eeeeee","showAxes":true,"showLabels":true},"parameters":\[{"id":"up1-4-3-ang","label":"Launch angle \\u03b8 above the horizontal","max":80,"min":10,"name":"ang","step":1,"unit":"degrees","value":75}\],"title":"Projectile range and the complementary launch angle","version":1,"viewport":{"centerX":230,"centerY":125,"scale":1.38}}#figure(figph[Figure a shows the trajectories of projectiles launched at the same initial 45 degree angle above the horizontal and different initial velocities. The trajectories are shown from launch to landing back at the initial elevation. In orange is the trajectory for 30 meters per second, giving a range R (distance from launch to landing) of 91.8 m. In purple is the trajectory for 40 meters per second, giving a range R of 163 m. In blue is the trajectory for 50 meters per second, giving a range R of 255 m. The maximum height of the projectile increases with initial speed. Figure b shows the trajectories of projectiles launched at the same initial speed of 50 meters per second and different launch angles. The trajectories are shown from launch to landing back at the initial elevation. In orange is the trajectory for an angle of 15 degrees above the horizontal, giving a range R of 128 m. In purple is the trajectory for an angle of 45 degrees above the horizontal, giving a range R of 255 m. In blue is the trajectory for an angle of 75 degrees above the horizontal, giving a range R of 128 m, the same as for the 15 degree trajectory. The maximum height increases with launch angle.], alt: "Figure a shows the trajectories of projectiles launched at the same initial 45 degree angle above the horizontal and different initial velocities. The trajectories are shown from launch to landing back at the initial elevation. In orange is the trajectory for 30 meters per second, giving a range R (distance from launch to landing) of 91.8 m. In purple is the trajectory for 40 meters per second, giving a range R of 163 m. In blue is the trajectory for 50 meters per second, giving a range R of 255 m. The maximum height of the projectile increases with initial speed. Figure b shows the trajectories of projectiles launched at the same initial speed of 50 meters per second and different launch angles. The trajectories are shown from launch to landing back at the initial elevation. In orange is the trajectory for an angle of 15 degrees above the horizontal, giving a range R of 128 m. In purple is the trajectory for an angle of 45 degrees above the horizontal, giving a range R of 255 m. In blue is the trajectory for an angle of 75 degrees above the horizontal, giving a range R of 128 m, the same as for the 15 degree trajectory. The maximum height increases with launch angle.", caption: [Trajectories of projectiles on level ground. (a) The greater the initial speed #math.equation(block: false, alt: "v sub 0 ,")[$v_(0) ,$] the greater the range for a given initial angle. (b) The effect of initial angle #math.equation(block: false, alt: "θ sub 0")[$θ_(0)$] on the range of a projectile with a given initial speed. Note that the range is the same for initial angles of #math.equation(block: false, alt: "15 °")[$15 "°"$] and #math.equation(block: false, alt: "75 ° ,")[$75 "°" ,$] although the maximum heights of those paths are different.]) #examplebox("Example 3")[Comparing Golf Shots][ A golfer encounters two different situations on different holes. On the second hole they are 120 m from the green and want to hit the ball 90 m and let it run onto the green. They angle the shot low to the ground at #math.equation(block: false, alt: "30 °")[$30 "°"$] to the horizontal to let the ball roll after impact. On the fourth hole they are 90 m from the green and want to let the ball drop with a minimum amount of rolling after impact. Here, they angle the shot at #math.equation(block: false, alt: "70 °")[$70 "°"$] to the horizontal to minimize rolling after impact. Both shots are hit and impacted on a level surface. (a) What is the initial speed of the ball at the second hole? (b) What is the initial speed of the ball at the fourth hole? (c) Write the trajectory equation for both cases. (d) Graph the trajectories. Strategy We see that the range equation has the initial speed and angle, so we can solve for the initial speed for both (a) and (b). When we have the initial speed, we can use this value to write the trajectory equation. Solution (a) #math.equation(block: false, alt: "R equals the fraction v 0 2 sin 2 θ sub 0 over g ⇒ v sub 0 equals the square root of the fraction R g over sin 2 θ sub 0 equals the square root of the fraction 90.0 m open parenthesis 9.8 m / s squared close parenthesis over sin open parenthesis 2 open parenthesis 30 ° close parenthesis close parenthesis equals 31.9 m / s")[$R = frac(v_(0)^(2) "sin" #h(0.1em) 2 θ_(0), g) ⇒ v_(0) = sqrt(frac(R g, "sin" #h(0.1em) 2 θ_(0))) = sqrt(frac(90.0 #h(0.2em) "m" ( 9.8 #h(0.2em) "m" "/" "s"^(2) ), "sin" ( 2 ( 30 "°" ) ))) = 31.9 #h(0.2em) "m" "/" "s"$] (b) #math.equation(block: false, alt: "R equals the fraction v 0 2 sin 2 θ sub 0 over g ⇒ v sub 0 equals the square root of the fraction R g over sin 2 θ sub 0 equals the square root of the fraction 90.0 m open parenthesis 9.8 m / s squared close parenthesis over sin open parenthesis 2 open parenthesis 70 ° close parenthesis close parenthesis equals 37.0 m / s")[$R = frac(v_(0)^(2) "sin" #h(0.1em) 2 θ_(0), g) ⇒ v_(0) = sqrt(frac(R g, "sin" #h(0.1em) 2 θ_(0))) = sqrt(frac(90.0 #h(0.2em) "m" ( 9.8 #h(0.2em) "m" "/" "s"^(2) ), "sin" ( 2 ( 70 "°" ) ))) = 37.0 #h(0.2em) "m" "/" "s"$] (c) #linebreak() #math.equation(block: false, alt: "y equals x [ tan θ sub 0 minus the fraction g over 2 open parenthesis v sub 0 cos θ sub 0 close parenthesis squared x ]; Second hole: y equals x [ tan 30 ° minus the fraction 9.8 m / s squared over 2 [ open parenthesis 31.9 m / s)( cos 30 ° close parenthesis ] squared x ] equals 0.58 x minus 0.0064 x squared; Fourth hole: y equals x [ tan 70 ° minus the fraction 9.8 m / s squared over 2 [ open parenthesis 37.0 m / s)( cos 70 ° close parenthesis ] squared x ] equals 2.75 x minus 0.0306 x squared")[$y = x [ "tan" #h(0.1em) θ_(0) − frac(g, 2 attach(( v_(0) "cos" #h(0.1em) θ_(0) ), t: 2)) x ] \ "Second hole:" #h(0.2em) y = x [ "tan" #h(0.2em) 30 "°" − frac(9.8 #h(0.2em) "m" "/" "s"^(2), 2 attach([ ( 31.9 #h(0.2em) "m" "/" "s)(" "cos" 30 "°" ) ], t: 2)) x ] = 0.58 x − 0.0064 x^(2) \ "Fourth hole:" #h(0.2em) y = x [ "tan" #h(0.2em) 70 "°" − frac(9.8 #h(0.2em) "m" "/" "s"^(2), 2 attach([ ( 37.0 #h(0.2em) "m" "/" "s)(" "cos" #h(0.2em) 70 "°" ) ], t: 2)) x ] = 2.75 x − 0.0306 x^(2)$] (d) Using a graphing utility, we can compare the two trajectories, which are shown. #figure(figph[Two parabolic functions are shown. The range for both trajectories is 90 meters. One shot travels much higher than the other. The higher shot has an initial velocity of 37 meters per second and an angle of 70 degrees. The lower shot has an initial velocity of 31.9 meters per second and an angle of 30 degrees.], alt: "Two parabolic functions are shown. The range for both trajectories is 90 meters. One shot travels much higher than the other. The higher shot has an initial velocity of 37 meters per second and an angle of 70 degrees. The lower shot has an initial velocity of 31.9 meters per second and an angle of 30 degrees.", caption: [Two trajectories of a golf ball with a range of 90 m. The impact points of both are at the same level as the launch point.]) Significance The initial speed for the shot at #math.equation(block: false, alt: "70 °")[$70 "°"$] is greater than the initial speed of the shot at #math.equation(block: false, alt: "30 ° .")[$30 "°" .$] Note from that if the two projectiles were launched at the same speed but at different angles, the projectiles would have the same range as long as the angles were less than #math.equation(block: false, alt: "90 ° .")[$90 "°" .$] The launch angles in this example add to give a number greater than #math.equation(block: false, alt: "90 ° .")[$90 "°" .$] Thus, the shot at #math.equation(block: false, alt: "70 °")[$70 "°"$] has to have a greater launch speed to reach 90 m, otherwise it would land at a shorter distance. ] #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ If the two golf shots in were launched at the same speed, which shot would have the greatest range? #solutionbox[ The golf shot at #math.equation(block: false, alt: "30 ° .")[$30 "°" .$] ] ] When we speak of the range of a projectile on level ground, we assume #emph[R] is very small compared with the circumference of Earth. If, however, the range is large, Earth curves away below the projectile and the acceleration resulting from gravity changes direction along the path. The range is larger than predicted by the range equation given earlier because the projectile has farther to fall than it would on level ground, as shown in , which is based on a drawing in Newton’s #emph[Principia.] If the initial speed is great enough, the projectile goes into orbit. Earth’s surface drops 5 m every 8000 m. In 1 s an object falls 5 m without air resistance. Thus, if an object is given a horizontal velocity of 8000 m/s (or 18,000 mi/hr) near Earth’s surface, it will go into orbit around the planet because the surface continuously falls away from the object. This is roughly the speed of the Space Shuttle in a low Earth orbit when it was operational, or any satellite in a low Earth orbit. These and other aspects of orbital motion, such as Earth’s rotation, are covered in greater depth in Gravitation. #figure(figph[The figure shows a drawing of the earth with a tall tower at the north pole and a horizontal arrow labeled v 0 pointing to the right. 5 trajectories that start at the top of the tower are shown. The first reaches the earth near the tower. The second reaches the earth farther from the tower, and the third even farther. The fourth trajectory hits the earth at the equator, and is tangent to the surface at the equator. The fifth trajectory is a circle concentric with the earth.], alt: "The figure shows a drawing of the earth with a tall tower at the north pole and a horizontal arrow labeled v 0 pointing to the right. 5 trajectories that start at the top of the tower are shown. The first reaches the earth near the tower. The second reaches the earth farther from the tower, and the third even farther. The fourth trajectory hits the earth at the equator, and is tangent to the surface at the equator. The fifth trajectory is a circle concentric with the earth.", caption: [Projectile to satellite. In each case shown here, a projectile is launched from a very high tower to avoid air resistance. With increasing initial speed, the range increases and becomes longer than it would be on level ground because Earth curves away beneath its path. With a speed of 8000 m/s, orbit is achieved.]) #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ At #link("https://openstax.org/l/21phetpromot")[PhET Explorations: Projectile Motion], learn about projectile motion in terms of the launch angle and initial velocity. ] === Summary - Projectile motion is the motion of an object subject only to the acceleration of gravity, where the acceleration is constant, as near the surface of Earth. - To solve projectile motion problems, we analyze the motion of the projectile in the horizontal and vertical directions using the one-dimensional kinematic equations for #emph[x] and #emph[y]. - The time of flight of a projectile launched with initial vertical velocity #math.equation(block: false, alt: "v sub 0 y")[$v_(0 y)$] on an even surface is given by #math.equation(block: true, alt: "T sub t o f equals the fraction 2 open parenthesis v sub 0 sin θ close parenthesis over g .")[$T_(t o f) = frac(2 ( v_(0) "sin" #h(0.1em) θ ), g) .$] This equation is valid only when the projectile lands at the same elevation from which it was launched. - The maximum horizontal distance traveled by a projectile is called the range. Again, the equation for range is valid only when the projectile lands at the same elevation from which it was launched. === Conceptual Questions Answer the following questions for projectile motion on level ground assuming negligible air resistance, with the initial angle being neither #math.equation(block: false, alt: "0 °")[$0 "°"$] nor #math.equation(block: false, alt: "90 ° :")[$90 "°" :$] (a) Is the velocity ever zero? (b) When is the velocity a minimum? A maximum? (c) Can the velocity ever be the same as the initial velocity at a time other than at #emph[t] = 0? (d) Can the speed ever be the same as the initial speed at a time other than at #emph[t] = 0? #solutionbox[ a. no; b. minimum at apex of trajectory and maximum at launch and impact; c. no, velocity is a vector; d. yes, where it lands ] Answer the following questions for projectile motion on level ground assuming negligible air resistance, with the initial angle being neither #math.equation(block: false, alt: "0 °")[$0 "°"$] nor #math.equation(block: false, alt: "90 ° :")[$90 "°" :$] (a) Is the acceleration ever zero? (b) Is the vector #math.equation(block: false, alt: "v →")[$arrow(v)$] ever parallel or antiparallel to the vector #math.equation(block: false, alt: "a →")[$arrow(a)$]? (c) Is the vector v ever perpendicular to the vector a? If so, where is this located? A dime is placed at the edge of a table so it hangs over slightly. A quarter is slid horizontally on the table surface perpendicular to the edge and hits the dime head on. Which coin hits the ground first? #solutionbox[ They both hit the ground at the same time. ] === Problems A bullet is shot horizontally from shoulder height (1.5 m) with an initial speed 200 m/s. (a) How much time elapses before the bullet hits the ground? (b) How far does the bullet travel horizontally? #solutionbox[ a. #math.equation(block: false, alt: "t equals 0.55 s")[$t = 0.55 #h(0.2em) "s"$], b. #math.equation(block: false, alt: "x equals 110 m")[$x = 110 #h(0.2em) "m"$] ] A marble rolls off a tabletop 1.0 m high and hits the floor at a point 3.0 m away from the table’s edge in the horizontal direction. (a) How long is the marble in the air? (b) What is the speed of the marble when it leaves the table’s edge? (c) What is its speed when it hits the floor? A dart is thrown horizontally at a speed of 10 m/s at the bull’s-eye of a dartboard 2.4 m away, as in the following figure. (a) How far below the intended target does the dart hit? (b) What does your answer tell you about how proficient dart players throw their darts? #figure(figph[An illustration of a person throwing a dart. The dart is released horizontally a distance of 2.4 meters from the dart board, level with the bulls eye of the dart board, with a speed of 10 meters per second.], alt: "An illustration of a person throwing a dart. The dart is released horizontally a distance of 2.4 meters from the dart board, level with the bulls eye of the dart board, with a speed of 10 meters per second.", caption: none) #solutionbox[ a. #math.equation(block: false, alt: "t equals 0.24 s, d equals 0.28 m")[$t = 0.24 "s," #h(0.5em) d = 0.28 #h(0.2em) "m"$], b. They aim high. #linebreak() #figure(figph[An illustration of a person throwing a dart. The dart is released horizontally a distance of 2.4 meters from the dart board, level with the bulls eye of the dart board, with a speed of 10 meters per second.], alt: "An illustration of a person throwing a dart. The dart is released horizontally a distance of 2.4 meters from the dart board, level with the bulls eye of the dart board, with a speed of 10 meters per second.", caption: none) ] An airplane flying horizontally with a speed of 500 km/h at a height of 800 m drops a crate of supplies (see the following figure). If the parachute fails to open, how far in front of the release point does the crate hit the ground? #figure(figph[An airplane releases a package. The airplane has a horizontal velocity of 500 kilometers per hour. The package’s trajectory is the right half of a downward-opening parabola, initially horizontal at the airplane and curving down until it hits the ground.], alt: "An airplane releases a package. The airplane has a horizontal velocity of 500 kilometers per hour. The package’s trajectory is the right half of a downward-opening parabola, initially horizontal at the airplane and curving down until it hits the ground.", caption: none) Suppose the airplane in the preceding problem fires a projectile horizontally in its direction of motion at a speed of 300 m/s relative to the plane. (a) How far in front of the release point does the projectile hit the ground? (b) What is its speed when it hits the ground? #solutionbox[ a., #math.equation(block: false, alt: "t equals 12.8 s, x equals 5619 m")[$t = 12.8 #h(0.2em) "s," #h(0.5em) x = 5619 #h(0.2em) "m"$] b. #math.equation(block: false, alt: "v sub y equals 125.0 m / s, v sub x equals 439.0 m / s, | v → | equals 456.0 m / s")[$v_(y) = 125.0 #h(0.2em) "m" "/" "s," #h(0.5em) v_(x) = 439.0 #h(0.2em) "m" "/" "s," #h(0.5em) | arrow(v) | = 456.0 #h(0.2em) "m" "/" "s"$] ] A fastball pitcher can throw a baseball at a speed of 40 m/s (90 mi/h). (a) Assuming the pitcher can release the ball 16.7 m from home plate so the ball is moving horizontally, how long does it take the ball to reach home plate? (b) How far does the ball drop between the pitcher’s hand and home plate? A projectile is launched at an angle of #math.equation(block: false, alt: "30 °")[$30 "°"$] and lands 20 s later at the same height as it was launched. (a) What is the initial speed of the projectile? (b) What is the maximum altitude? (c) What is the range? (d) Calculate the displacement from the point of launch to the position on its trajectory at 15 s. #solutionbox[ a. #math.equation(block: false, alt: "v sub y equals v sub 0 y minus g t , t equals 10 s, v sub y equals 0 , v sub 0 y equals 98.0 m / s , v sub 0 equals 196.0 m / s")[$v_(y) = v_(0 y) − g t , #h(0.5em) t = 10 "s," #h(0.5em) v_(y) = 0 , #h(0.5em) v_(0 y) = 98.0 #h(0.2em) "m" "/" "s" , #h(0.5em) v_(0) = 196.0 #h(0.2em) "m" "/" "s"$], #linebreak() b. #math.equation(block: false, alt: "h equals 490.0 m ,")[$h = 490.0 #h(0.2em) "m" ,$] #linebreak() c. #math.equation(block: false, alt: "v sub 0 x equals 169.7 m / s, x equals 3394.0 m,")[$v_(0 x) = 169.7 #h(0.2em) "m" "/" "s," #h(0.5em) x = 3394.0 #h(0.2em) "m,"$] #linebreak() d. #math.equation(block: false, alt: "x equals 169.7 m/s open parenthesis 15.0 s close parenthesis equals 2550 m; y equals open parenthesis 98.0 m/s close parenthesis open parenthesis 15.0 s close parenthesis minus 4.9 open parenthesis 15.0 s close parenthesis squared equals 368 m; s r equals 2550 m i ^ plus 368 m j ^")[$x = 169.7 #h(0.2em) "m/s" #h(0.2em) ( 15.0 #h(0.2em) "s" ) = 2550 #h(0.2em) "m" \ y = ( 98.0 #h(0.2em) "m/s" #h(0.2em) ) #h(0.2em) ( 15.0 #h(0.2em) "s" ) − 4.9 ( 15.0 "s" )^(2) = 368 #h(0.2em) "m" \ accent(s, "r") = 2550 #h(0.2em) "m" hat(i) + 368 #h(0.2em) "m" hat(j)$] ] A basketball player shoots toward a basket 6.1 m away and 3.0 m above the floor. If the ball is released 1.8 m above the floor at an angle of #math.equation(block: false, alt: "60 °")[$60 "°"$] above the horizontal, what must the initial speed be if it were to go through the basket? At a particular instant, a hot air balloon is 100 m in the air and descending at a constant speed of 2.0 m/s. At this exact instant, a girl throws a ball horizontally, relative to herself, with an initial speed of 20 m/s. When she lands, where will she find the ball? Ignore air resistance. #solutionbox[ #math.equation(block: true, alt: "−100 m equals open parenthesis −2.0 m / s close parenthesis t minus open parenthesis 4.9 m / s squared close parenthesis t squared ,")[$−100 #h(0.2em) "m" = ( −2.0 #h(0.2em) "m" "/" "s" ) t − ( 4.9 #h(0.2em) "m" "/" "s"^(2) ) t^(2) ,$] #math.equation(block: true, alt: "t equals 4.3 s,")[$t = 4.3 #h(0.2em) "s,"$]#math.equation(block: true, alt: "x equals 86.0 m")[$x = 86.0 #h(0.2em) "m"$] ] A man on a motorcycle traveling at a uniform speed of 10 m/s throws an empty can straight upward relative to himself with an initial speed of 3.0 m/s. Find the equation of the trajectory as seen by a police officer on the side of the road. Assume the initial position of the can is the point where it is thrown. Ignore air resistance. An athlete can jump a distance of 8.0 m in the broad jump. What is the maximum distance the athlete can jump on the Moon, where the gravitational acceleration is one-sixth that of Earth? #solutionbox[ #math.equation(block: true, alt: "R sub M o o n equals 48 m")[$R_(M o o n) = 48 #h(0.2em) "m"$] ] The maximum horizontal distance a boy can throw a ball is 50 m. Assume he can throw with the same initial speed at all angles. How high does he throw the ball when he throws it straight upward? A rock is thrown off a cliff at an angle of #math.equation(block: false, alt: "53 °")[$53 "°"$] with respect to the horizontal. The cliff is 100 m high. The initial speed of the rock is 30 m/s. (a) How high above the edge of the cliff does the rock rise? (b) How far has it moved horizontally when it is at maximum altitude? (c) How long after the release does it hit the ground? (d) What is the range of the rock? (e) What are the horizontal and vertical positions of the rock relative to the edge of the cliff at #emph[t] = 2.0 s, #emph[t] = 4.0 s, and #emph[t] = 6.0 s? #solutionbox[ a. $$#math.equation(block: false, alt: "v sub 0 y equals 24 m / s")[$v_(0 y) = 24 #h(0.2em) "m" "/" "s"$]#math.equation(block: false, alt: "v y 2 equals v 0 y 2 minus 2 g y ⇒ h equals 29.3 m")[$v_(y)^(2) = v_(0 y)^(2) − 2 g y ⇒ h = 29.3 #h(0.2em) "m"$], #linebreak() b. #math.equation(block: false, alt: "t equals 2.4 s v sub 0 x equals 18 m/s x equals 43.2 m")[$t = 2.4 #h(0.2em) "s" #h(0.5em) v_(0 x) = 18 #h(0.2em) "m/s" #h(0.5em) x = 43.2 #h(0.2em) "m"$], #linebreak() c. #math.equation(block: false, alt: "y equals −100 m y sub 0 equals 0")[$y = −100 #h(0.2em) "m" #h(0.5em) y_(0) = 0$] #math.equation(block: false, alt: "y minus y sub 0 equals v sub 0 y t minus the fraction 1 over 2 g t squared minus 100 equals 24 t minus 4.9 t squared")[$y − y_(0) = v_(0 y) t − frac(1, 2) g t^(2) #h(0.5em) − 100 = 24 t − 4.9 t^(2)$] #math.equation(block: false, alt: "⇒ t equals 7.58 s")[$⇒ t = 7.58 #h(0.2em) "s"$], #linebreak() d. #math.equation(block: false, alt: "x equals 136.44 m")[$x = 136.44 #h(0.2em) "m"$], #linebreak() e. #math.equation(block: false, alt: "t equals 2.0 s y equals 28.4 m x equals 36 m")[$t = 2.0 #h(0.2em) "s" #h(0.5em) y = 28.4 #h(0.2em) "m" #h(0.5em) x = 36 #h(0.2em) "m"$] #linebreak() #math.equation(block: false, alt: "t equals 4.0 s y equals 17.6 m x equals 72 m")[$t = 4.0 #h(0.2em) "s" #h(0.5em) y = 17.6 #h(0.2em) "m" #h(0.5em) x = 72 #h(0.2em) "m"$] #linebreak() #math.equation(block: false, alt: "t equals 6.0 s y equals −32.4 m x equals 108 m")[$t = 6.0 #h(0.2em) "s" #h(0.5em) y = −32.4 #h(0.2em) "m" #h(0.5em) x = 108 #h(0.2em) "m"$] ] Trying to escape his pursuers, a secret agent skis off a slope inclined at #math.equation(block: false, alt: "30 °")[$30 "°"$] below the horizontal at 60 km/h. To survive and land on the snow 100 m below, he must clear a gorge 60 m wide. Does he make it? Ignore air resistance. #figure(figph[A skier is moving with velocity v sub 0 down a slope that is inclined at 30 degrees to the horizontal. The skier is at the edge of a 60 m wide gap. The other side of the gap is 100 m lower.], alt: "A skier is moving with velocity v sub 0 down a slope that is inclined at 30 degrees to the horizontal. The skier is at the edge of a 60 m wide gap. The other side of the gap is 100 m lower.", caption: none) A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of #math.equation(block: false, alt: "40 °")[$40 "°"$] with an initial speed of 20 m/s, how close to the green does she come? #solutionbox[ #math.equation(block: false, alt: "v sub 0 y equals 12.9 m / s y minus y sub 0 equals v sub 0 y t minus the fraction 1 over 2 g t squared minus 20.0 equals 12.9 t minus 4.9 t squared")[$v_(0 y) = 12.9 #h(0.2em) "m" "/" "s" #h(0.2em) y − y_(0) = v_(0 y) t − frac(1, 2) g t^(2) #h(0.5em) − 20.0 = 12.9 t − 4.9 t^(2)$] #linebreak() #math.equation(block: false, alt: "t equals 3.7 s v sub 0 x equals 15.3 m / s ⇒ x equals 56.7 m")[$t = 3.7 #h(0.2em) "s" #h(0.5em) v_(0 x) = 15.3 #h(0.2em) "m" "/" "s" ⇒ x = 56.7 #h(0.2em) "m"$] #linebreak() So the golfer’s shot lands 13.3 m short of the green. ] A projectile is shot at a hill, the base of which is 300 m away. The projectile is shot at #math.equation(block: false, alt: "60 °")[$60 "°"$] above the horizontal with an initial speed of 75 m/s. The hill can be approximated by a plane sloped at #math.equation(block: false, alt: "20 °")[$20 "°"$] to the horizontal. Relative to the coordinate system shown in the following figure, the equation of this straight line is #math.equation(block: false, alt: "y equals open parenthesis tan 20 ° close parenthesis x minus 109 .")[$y = ( "tan" 20 "°" ) x − 109 .$] Where on the hill does the projectile land? #figure(figph[A projectile is shot from the origin at a hill, the base of which is 300 m away. The projectile is shot at 60 degrees above the horizontal with an initial speed of 75 m/s. The hill is sloped away from the origin at 20 degrees to the horizontal. The slope is expressed as the equation y equals (tan of 20 degrees) times x minus 109.], alt: "A projectile is shot from the origin at a hill, the base of which is 300 m away. The projectile is shot at 60 degrees above the horizontal with an initial speed of 75 m/s. The hill is sloped away from the origin at 20 degrees to the horizontal. The slope is expressed as the equation y equals (tan of 20 degrees) times x minus 109.", caption: none) An astronaut on Mars kicks a soccer ball at an angle of #math.equation(block: false, alt: "45 °")[$45 "°"$] with an initial velocity of 15 m/s. If the acceleration of gravity on Mars is #math.equation(block: false, alt: "3.7 m/ s squared")[$3.7 "m/" "s"^(2)$], (a) what is the range of the soccer kick on a flat surface? (b) What would be the range of the same kick on the Moon, where gravity is one-sixth that of Earth? #solutionbox[ a. #math.equation(block: false, alt: "R equals 60.8 m")[$R = 60.8 #h(0.2em) "m"$], #linebreak() b. #math.equation(block: false, alt: "R equals 137.8 m")[$R = 137.8 #h(0.2em) "m"$] ] Mike Powell holds the record for the long jump of 8.95 m, established in 1991. If he left the ground at an angle of #math.equation(block: false, alt: "15 ° ,")[$15 "°" ,$] what was his initial speed? MIT’s robot cheetah can jump over obstacles 46 cm high and has speed of 12.0 km/h. (a) If the robot launches itself at an angle of #math.equation(block: false, alt: "60 °")[$60 "°"$] at this speed, what is its maximum height? (b) What would the launch angle have to be to reach a height of 46 cm? #solutionbox[ a. #math.equation(block: false, alt: "v y 2 equals v 0 y 2 minus 2 g y ⇒ y equals 2.9 m / s")[$v_(y)^(2) = v_(0 y)^(2) − 2 g y ⇒ y = 2.9 #h(0.2em) "m" "/" "s"$] #linebreak() #math.equation(block: false, alt: "y equals 3.3 m / s")[$y = 3.3 #h(0.2em) "m" "/" "s"$] #linebreak() #math.equation(block: false, alt: "y equals the fraction v 0 y 2 over 2 g equals the fraction open parenthesis v sub 0 sin θ close parenthesis squared over 2 g ⇒ sin θ equals 0.91 ⇒ θ equals 65.5 °")[$y = frac(v_(0 y)^(2), 2 g) = frac(attach(( v_(0) "sin" #h(0.1em) θ ), t: 2), 2 g) ⇒ "sin" #h(0.1em) θ = 0.91 ⇒ θ = 65.5 "°"$] ] Mt. Asama, Japan, is an active volcano. In 2009, an eruption threw solid volcanic rocks that landed 1 km horizontally from the crater. If the volcanic rocks were launched at an angle of #math.equation(block: false, alt: "40 °")[$40 "°"$] with respect to the horizontal and landed 900 m below the crater, (a) what would be their initial velocity and (b) what is their time of flight? Drew Brees of the New Orleans Saints can throw a football 23.0 m/s (50 mph). If he angles the throw at #math.equation(block: false, alt: "10 °")[$10 "°"$] from the horizontal, what distance does it go if it is to be caught at the same elevation as it was thrown? #solutionbox[ #math.equation(block: true, alt: "R equals 18.5 m")[$R = 18.5 #h(0.2em) "m"$] ] The Lunar Roving Vehicle used in NASA’s late #emph[Apollo] missions reached an unofficial lunar land speed of #math.equation(block: false, alt: "5.0 m/s")[$5.0 #h(0.2em) "m/s"$] by astronaut Eugene Cernan. If the rover was moving at this speed on a flat lunar surface and hit a small bump that projected it off the surface at an angle of #math.equation(block: false, alt: "20 ° ,")[$20 "°" ,$] how long would it be “airborne” on the Moon? A soccer goal is 2.44 m high. A player kicks the ball at a distance 10 m from the goal at an angle of #math.equation(block: false, alt: "25 ° .")[$25 "°" .$] The ball hits the crossbar at the top of the goal. What is the initial speed of the soccer ball? #solutionbox[ #math.equation(block: true, alt: "y equals open parenthesis tan θ sub 0 close parenthesis x minus [ the fraction g over 2 open parenthesis v sub 0 cos θ sub 0 close parenthesis squared ] x squared ⇒ v sub 0 equals 16.4 m / s")[$y = ( "tan" #h(0.1em) θ_(0) ) x − [ frac(g, 2 attach(( v_(0) "cos" #h(0.1em) θ_(0) ), t: 2)) ] x^(2) ⇒ v_(0) = 16.4 #h(0.2em) "m" "/" "s"$] ] Olympus Mons on Mars is the largest volcano in the solar system, at a height of 25 km and with a radius of 312 km. If you are standing on the summit, with what initial velocity would you have to fire a projectile from a cannon horizontally to clear the volcano and land on the surface of Mars? Note that Mars has an acceleration of gravity of #math.equation(block: false, alt: "3.7 m / s squared .")[$3.7 #h(0.2em) "m" "/" "s"^(2) .$] In 1999, Robbie Knievel was the first to jump the Grand Canyon on a motorcycle. At a narrow part of the canyon (69.0 m wide) and traveling 35.8 m/s off the takeoff ramp, he reached the other side. What was his launch angle? #solutionbox[ #math.equation(block: true, alt: "R equals the fraction v 0 2 sin 2 θ sub 0 over g ⇒ θ sub 0 equals 15.9 °")[$R = frac(v_(0)^(2) "sin" #h(0.2em) 2 θ_(0), g) ⇒ θ_(0) = 15.9 "°"$] ] You throw a baseball at an initial speed of 15.0 m/s at an angle of #math.equation(block: false, alt: "30 °")[$30 "°"$] with respect to the horizontal. What would the ball’s initial speed have to be at #math.equation(block: false, alt: "30 °")[$30 "°"$] on a planet that has twice the acceleration of gravity as Earth to achieve the same range? Consider launch and impact on a horizontal surface. Aaron Rodgers throws a football at 20.0 m/s to his wide receiver, who is running straight down the field at 9.4 m/s. If Aaron throws the football when the wide receiver is 10.0 m in front of him, (a) at what angle does Aaron have to launch the ball so the ball will be at the same height as the receiver when the receiver makes it to 20.0 m in front of Aaron? (b) Will the receiver be able to catch the ball? #solutionbox[ (a) It takes the wide receiver 1.1 s to cover the last 10 m of his run. #linebreak() #math.equation(block: false, alt: "T sub tof equals the fraction 2 open parenthesis v sub 0 sin θ close parenthesis over g ⇒ sin θ equals 0.27 ⇒ θ equals 15.6 °")[$T_("tof") = frac(2 ( v_(0) "sin" #h(0.1em) θ ), g) ⇒ "sin" #h(0.1em) θ = 0.27 ⇒ θ = 15.6 "°"$] #linebreak() (b) #math.equation(block: false, alt: "x equals the fraction v 0 2 sin open parenthesis 2 θ close parenthesis over g equals 21 m")[$x = frac(v_(0)^(2) sin ( 2 θ ), g) = 21 upright(m)$] #linebreak() Therefore, the ball will be overthrown, and the receiver will not be able to catch it. ]