Projectile Motion

SECTION LEARNING OBJECTIVES

By the end of this section, you will be able to do the following:

• Describe the properties of projectile motion
• Apply kinematic equations and vectors to solve problems involving projectile motion

Section Key Terms

air resistance	maximum height (of a projectile)	projectile
projectile motion	range	trajectory

Properties of Projectile Motion

Projectile motion is the motion of an object thrown (projected) into the air when, after the initial force that launches the object, air resistance is negligible and the only other force that object experiences is the force of gravity. The object is called a projectile, and its path is called its trajectory. Air resistance is a frictional force that slows its motion and can significantly alter the trajectory of the motion. Due to the difficulty in calculation, only situations in which the deviation from projectile motion is negligible and air resistance can be ignored are considered in introductory physics. That approximation is often quite accurate.

The most important concept in projectile motion is that when air resistance is ignored, horizontal and vertical motions are independent, meaning that they don’t influence one another. Figure 5.27 compares a cannonball in free fall (in blue) to a cannonball launched horizontally in projectile motion (in red). You can see that the cannonball in free fall falls at the same rate as the cannonball in projectile motion. Keep in mind that if the cannon launched the ball with any vertical component to the velocity, the vertical displacements would not line up perfectly.

Since vertical and horizontal motions are independent, we can analyze them separately, along perpendicular axes. To do this, we separate projectile motion into the two components of its motion, one along the horizontal axis and the other along the vertical.

Figure 5.27 The diagram shows the projectile motion of a cannonball shot at a horizontal angle versus one dropped with no horizontal velocity. Note that both cannonballs have the same vertical position over time.

We’ll call the horizontal axis the x-axis and the vertical axis the y-axis. For notation, d is the total displacement, and x and y are its components along the horizontal and vertical axes. The magnitudes of these vectors are x and y, as illustrated in Figure 5.28.

Figure 5.28 A boy kicks a ball at angle θ, and it is displaced a distance of s along its trajectory.

As usual, we use velocity, acceleration, and displacement to describe motion. We must also find the components of these variables along the x– and y-axes. The components of acceleration are then very simple a_y = –g = –9.80 m/s². Note that this definition defines the upwards direction as positive. Because gravity is vertical, a_x = 0. Both accelerations are constant, so we can use the kinematic equations. For review, the kinematic equations from a previous chapter are summarized in Table 5.1.

x=x0+vavgt𝑥=𝑥0+𝑣𝑎𝑣𝑔𝑡 (when a=constant𝑎=constant )

vavg=v0+v2𝑣𝑎𝑣𝑔=𝑣0+𝑣2 (when a=0𝑎=0 )

v=v0+at𝑣=𝑣0+𝑎𝑡

x=x0+v0t+12at2𝑥=𝑥0+𝑣0𝑡+12𝑎𝑡2

v2=v20+2a(x−x0)𝑣2=𝑣02+2𝑎(𝑥−𝑥0)

Where x is position, x₀ is initial position, v is velocity, v_avg is average velocity, t is time and a is acceleration.

Solve Problems Involving Projectile Motion

The following steps are used to analyze projectile motion:

1. Separate the motion into horizontal and vertical components along the x- and y-axes. These axes are perpendicular, so Ax=Acosθ𝐴𝑥=𝐴cos𝜃 and Ay=Asinθ𝐴𝑦=𝐴sin𝜃 are used. The magnitudes of the displacement ss along x- and y-axes are called x𝑥 and y.𝑦. The magnitudes of the components of the velocity vv are vx=v​​​cosθ𝑣𝑥=𝑣​​​cos𝜃 and vy=v​​​sinθ𝑣𝑦=𝑣​​​sin𝜃, where v𝑣 is the magnitude of the velocity and θ𝜃 is its direction. Initial values are denoted with a subscript 0.
2. Treat the motion as two independent one-dimensional motions, one horizontal and the other vertical. The kinematic equations for horizontal and vertical motion take the following formsHorizontal Motion(ax=0)x=x0+vxtvx=v0x=vx=velocity  is a constant.Horizontal Motion(𝑎𝑥=0)𝑥=𝑥0+𝑣𝑥𝑡𝑣𝑥=𝑣0𝑥=vx=velocity  is a constant.Vertical motion (assuming positive is up ay=−g=−9.80 m/s2𝑎𝑦=−𝑔=−9.80 m/s2 )yvyyv2y====y0+12(v0y+vy)tv0y−gty0+v0yt−12gt2v20y−2g(y−y0)𝑦=𝑦0+12(𝑣0𝑦+𝑣𝑦)𝑡𝑣𝑦=𝑣0𝑦−𝑔𝑡𝑦=𝑦0+𝑣0𝑦𝑡−12𝑔𝑡2𝑣𝑦2=𝑣0𝑦2−2𝑔(𝑦−𝑦0)
3. 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 t𝑡 . The problem solving procedures here are the same as for one-dimensional kinematics.
4. Recombine the two motions to find the total displacement ss and velocity vv . We can use the analytical method of vector addition, which uses A=Ax2+Ay2−−−−−−−−√𝐴=𝐴𝑥2+𝐴𝑦2 and θ=tan−1(Ay/Ax)𝜃=tan−1(𝐴𝑦/𝐴𝑥) to find the magnitude and direction of the total displacement and velocity.Displacementd=x2+y2−−−−−−√θ=tan−1(y/x)Velocityv=v2x+v2y−−−−−−√θv=tan−1(vy/vx)Displacement𝑑=𝑥2+𝑦2𝜃=tan−1(𝑦/𝑥)Velocity𝑣=𝑣𝑥2+𝑣𝑦2𝜃𝑣=tan−1(𝑣𝑦/𝑣𝑥)θ𝜃 is the direction of the displacement dd, and θv𝜃v is the direction of the velocity vv . (See Figure 5.29Figure 5.29 (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  ax=0 𝑎𝑥=0 and  vx 𝑣𝑥 is thus 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 towards the Earth again, the vertical velocity increases again in magnitude but points in the opposite direction to the initial vertical velocity. (d) The x– and y-motions are recombined to give the total velocity at any given point on the trajectory.

TIPS FOR SUCCESS

For problems of projectile motion, it is important to set up a coordinate system. The first step is to choose an initial position for x𝑥 and y𝑦 . Usually, it is simplest to set the initial position of the object so that x0=0𝑥0=0 and y0=0𝑦0=0 .

Projectile at an Angle

This video presents an example of finding the displacement (or range) of a projectile launched at an angle. It also reviews basic trigonometry for finding the sine, cosine and tangent of an angle.

Assume the ground is uniformly level. If the horizontal component of a projectile’s velocity is doubled, but the vertical component is unchanged, what is the effect on the time of flight?

1. The time to reach the ground would remain the same since the vertical component is unchanged.
2. The time to reach the ground would remain the same since the vertical component of the velocity also gets doubled.
3. The time to reach the ground would be halved since the horizontal component of the velocity is doubled.
4. The time to reach the ground would be doubled since the horizontal component of the velocity is doubled.

WORKED EXAMPLE

A Fireworks Projectile Explodes High and Away

During a fireworks display like the one illustrated in Figure 5.30, a shell is shot into the air with an initial speed of 70.0 m/s at an angle of 75° above the horizontal. 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 passed between the launch of the shell and the explosion? (c) What is the horizontal displacement of the shell when it explodes?

Figure 5.30 The diagram shows the trajectory of a fireworks shell.

STRATEGY

The motion can be broken into horizontal and vertical motions in which ax=0𝑎𝑥=0 and  ay=g 𝑎𝑦=𝑔 . We can then define x0𝑥0 and y0𝑦0 to be zero and solve for the maximum height.

Discussion for (a)

Since up is positive, the initial velocity and maximum height are positive, but the acceleration due to gravity is negative. The maximum height depends only on the vertical component of the initial velocity. The numbers in this example are reasonable for large fireworks displays, the shells of which do reach such heights before exploding.

Discussion for (b)

This time is also reasonable for large fireworks. When you are able to see the launch of fireworks, you will notice several seconds pass before the shell explodes. Another way of finding the time is by using y=y0+v0yt−12gt2𝑦=𝑦0+𝑣0𝑦𝑡−12𝑔𝑡2, and solving the quadratic equation for t𝑡 .

Discussion for (c)

The 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. Once the shell explodes, air resistance has a major effect, and many fragments will land directly below, while some of the fragments may now have a velocity in the –x direction due to the forces of the explosion.

The expression we found for y𝑦 while solving part (a) of the previous problem works for any projectile motion problem where air resistance is negligible. Call the maximum height y=h𝑦=ℎ; then,

h=v20y2g.ℎ=𝑣0𝑦22𝑔.

This equation defines the maximum height of a projectile. The maximum height depends only on the vertical component of the initial velocity.

WORKED EXAMPLE

Calculating Projectile Motion: Hot Rock Projectile

Suppose a large rock is ejected from a volcano, as illustrated in Figure 5.31, with a speed of 25.0 m/s25.0 m/s and at an angle 35°35° above the horizontal. The rock strikes the side of the volcano at an altitude 20.0 m lower than its starting point. (a) Calculate the time it takes the rock to follow this path.

Figure 5.31 The diagram shows the projectile motion of a large rock from a volcano.

STRATEGY

Breaking this two-dimensional motion into two independent one-dimensional motions will allow us to solve for the time. The time a projectile is in the air depends only on its vertical motion.

Discussion

The time for projectile motion is completely determined by the vertical motion. So any projectile that has an initial vertical velocity of 14.3m/s14.3m/s and lands 20.0 m below its starting altitude will spend 3.96 s in the air.

Practice Problems

11.

If an object is thrown horizontally, travels with an average x-component of its velocity equal to 5m/s5m/s, and does not hit the ground, what will be the x-component of the displacement after 20s20s?

1. −100m−100m
2. −4m−4m
3. 4m4m
4. 100m100m

12.

If a ball is thrown straight up with an initial velocity of 20m/s20m/s upward, what is the maximum height it will reach?

1. −20.4m−20.4m
2. −1.02m−1.02m
3. 1.02m1.02m
4. 20.4m20.4m

The fact that vertical and horizontal motions are independent of each other lets us predict the range of a projectile. The range is the horizontal distance R traveled by a projectile on level ground, as illustrated in Figure 5.32. Throughout history, people have been interested in finding the range of projectiles for practical purposes, such as aiming cannons.

Figure 5.32 Trajectories of projectiles on level ground. (a) The greater the initial speed v0𝑣0, the greater the range for a given initial angle. (b) The effect of initial angle θ0𝜃0 on the range of a projectile with a given initial speed. Note that any combination of trajectories that add to 90 degrees will have the same range in the absence of air resistance, although the maximum heights of those paths are different.

How does the initial velocity of a projectile affect its range? Obviously, the greater the initial speed v0𝑣0, the greater the range, as shown in the figure above. The initial angle θ0𝜃0 also has a dramatic effect on the range. When air resistance is negligible, the range R𝑅 of a projectile on level ground is

R=v20sin2θ0g,𝑅=𝑣02sin2𝜃0𝑔,

where v0𝑣0 is the initial speed and θ0𝜃0 is the initial angle relative to the horizontal. It is important to note that the range doesn’t apply to problems where the initial and final y position are different, or to cases where the object is launched perfectly horizontally.

VIRTUAL PHYSICS

Projectile Motion

In this simulation you will learn about projectile motion by blasting objects out of a cannon. You can choose between objects such as a tank shell, a golf ball or even a Buick. Experiment with changing the angle, initial speed, and mass, and adding in air resistance. Make a game out of this simulation by trying to hit the target.

https://openstax.org/l/28prjctilemtion

Consider the simulation. If a projectile is launched on level ground, what launch angle maximizes the range of the projectile?

1. 0∘0∘
2. 30∘30∘
3. 45∘45∘
4. 60∘60∘

Check Your Understanding

13.

What is projectile motion?

1. Projectile motion is the motion of an object projected into the air and moving under the influence of gravity.
2. Projectile motion is the motion of an object projected into the air and moving independently of gravity.
3. Projectile motion is the motion of an object projected vertically upward into the air and moving under the influence of gravity.
4. Projectile motion is the motion of an object projected horizontally into the air and moving independently of gravity.

14.

What is the force experienced by a projectile after the initial force that launched it into the air in the absence of air resistance?

1. The nuclear force
2. The gravitational force
3. The electromagnetic force
4. The contact force