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
|air resistance||maximum height (of a projectile)||projectile|
Properties of Projectile Motion
Properties of Projectile Motion
Projectile motion is the motion of an object thrown (projected) into the air. After the initial force that launches the object, it only experiences the force of gravity. The object is called a projectile, and its path is called its trajectory. As an object travels through the air, it encounters a frictional force that slows its motion called air resistance. Air resistance does significantly alter trajectory motion, but due to the difficulty in calculation, it is ignored in introductory physics.
The most important concept in projectile motion is that horizontal and vertical motions are independent, meaning that they don’t influence one another. Figure 5.28 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.
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 5.29.
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 ay = –g = –9.80 m/s2. Note that this definition defines the upwards direction as positive. Because gravity is vertical, ax = 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.
Where x is position, x0 is initial position, v is velocity, vavg is average velocity, t is time and a is acceleration.
Solve Problems Involving Projectile Motion
Solve Problems Involving Projectile Motion
The following steps are used to analyze projectile motion:
- Separate the motion into horizontal and vertical components along the x- and y-axes. These axes are perpendicular, so and are used. The magnitudes of the displacementalong x- and y-axes are calledandThe magnitudes of the components of the velocityareand, whereis the magnitude of the velocity andis its direction. Initial values are denoted with a subscript 0.
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 forms
- 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. The problem solving procedures here are the same as for one-dimensional kinematics.
- Recombine the two motions to find the total displacementand velocity. We can use the analytical method of vector addition, which usesandto find the magnitude and direction of the total displacement and velocity.
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 and . Usually, it is simplest to set the initial position of the object so that and .
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.
- The time to reach the ground would remain the same since the vertical component is unchanged.
- The time to reach the ground would remain the same since the vertical component of the velocity also gets doubled.
- The time to reach the ground would be halved since the horizontal component of the velocity is doubled.
- The time to reach the ground would be doubled since the horizontal component of the velocity is doubled.
A Fireworks Projectile Explodes High and Away
During a fireworks display like the one illustrated in Figure 5.31, 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?
The motion can be broken into horizontal and vertical motions in whichand. We can then defineandto be zero and solve for the maximum height.
By height we mean the altitude or vertical position above the starting point. The highest point in any trajectory, the maximum height, is reached when; this is the moment when the vertical velocity switches from positive (upwards) to negative (downwards). Since we know the initial velocity, initial position, and the value of vy when the firework reaches its maximum height, we use the following equation to find
Because and are both zero, the equation simplifies to
Solving for gives
Now we must find , the component of the initial velocity in the y-direction. It is given by , where is the initial velocity of 70.0 m/s, andis the initial angle. Thus,
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.
There is more than one way to solve for the time to the highest point. In this case, the easiest method is to use . Because is zero, this equation reduces to
Note that the final vertical velocity, , at the highest point is zero. Therefore,
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, and solving the quadratic equation for.
Because air resistance is negligible, and the horizontal velocity is constant. The horizontal displacement is horizontal velocity multiplied by time as given by , where is equal to zero
where is the x-component of the velocity, which is given by Now,
The time for both motions is the same, and so is
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 while solving part (a) of the previous problem works for any projectile motion problem where air resistance is negligible. Call the maximum height ; then,
This equation defines the maximum height of a projectile. The maximum height depends only on the vertical component of the initial velocity.
Calculating Projectile Motion: Hot Rock Projectile
Suppose a large rock is ejected from a volcano, as illustrated in Figure 5.32, with a speed ofand at an angleabove 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.
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.
While the rock is in the air, it rises and then falls to a final position 20.0 m lower than its starting altitude. We can find the time for this by using
If we take the initial position to be zero, then the final position is Now the initial vertical velocity is the vertical component of the initial velocity, found from
Substituting known values yields
Rearranging terms gives a quadratic equation in
This expression is a quadratic equation of the form, where the constants are a = 4.90, b = –14.3, and c = –20.0. Its solutions are given by the quadratic formula
This equation yields two solutions t = 3.96 and t = –1.03. You may verify these solutions as an exercise. The time is t = 3.96 s or –1.03 s. The negative value of time implies an event before the start of motion, so we discard it. Therefore,
The time for projectile motion is completely determined by the vertical motion. So any projectile that has an initial vertical velocity ofand lands 20.0 m below its starting altitude will spend 3.96 s in the air.
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 5.33. Throughout history, people have been interested in finding the range of projectiles for practical purposes, such as aiming cannons.
How does the initial velocity of a projectile affect its range? Obviously, the greater the initial speed, the greater the range, as shown in the figure above. The initial anglealso has a dramatic effect on the range. When air resistance is negligible, the rangeof a projectile on level ground is
whereis the initial speed andis 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.
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.
Check Your Understanding
Check Your Understanding
- Projectile motion is the motion of an object projected into the air, which moves under the influence of gravity.
- Projectile motion is the motion of an object projected into the air which moves independently of gravity.
- Projectile motion is the motion of an object projected vertically upward into the air which moves under the influence of gravity.
- Projectile motion is the motion of an object projected horizontally into the air which moves independently of gravity.
What is the force experienced by a projectile after the initial force that launched it into the air in the absence of air resistance?
- The nuclear force
- The gravitational force
- The electromagnetic force
- The contact force