Making Predictions to Accurately Guide the Projectile to the Target

The Idea

In this experiment, you get to shoot things around the room. You can use a toy bow-and-arrow, a toy ping-pong ball shooter, a Nerf gun, a marble launcher, or a precision projectile launcher made for this purpose. You learn to make predictions that accurately guide the projectile to the target. In this case, using the laws of physics is not cheating. It does, however, give you a definite advantage compared with someone who is not armed with this knowledge.

First, you measure what is the best angle to aim something for it to travel the greatest distance.

Then, you make and test predictions. To hit a target, you need to know only two things: the velocity of the projectile and the angle at which it is shot. That's all. Knowing only those two conditions, you can determine how far the projectile will go, and how high it will go. The angle is easy to measure directly, so we will first work on a simple way to determine the velocity.

What You Need

– A projectile launcher, such as shown in Figure 9-1. Plastic rather than steel balls are safer.

– Or, a toy gun, a toy bow-and-arrow, a ping-pong ball shooter, Nerf gun, or a marble launcher.

• projectile and launcher
• tape measure
• protractor
• target—horizontal: pan or cup; vertical: ring on a ring stand
• stool(s) or other moveable object to hold the target at the height of the launcher

Method

What is the best angle?

We start here because this part does not involve any number crunching.

1. You will be shooting your projectile from ground-to-ground or from table top to raised surface at the same height as the table top. The projectile should be launched and land at the same height.
2. Select a setting for your launcher that will fire a projectile from a given height and return to that same height without hitting the ceiling, a wall, or breaking anything.
3. For every test in this part, you will be using the same velocity. Pick an angle to shoot the projectile at. Launch the projectile and measure the distance. Increase or decrease the launch angle until you find the angle that gives the greatest distance. (Remember, for this part, we are measuring the distance the object goes after returning to the same height from which it was launched.)

Determine the velocity of the launcher (to make predictions).

For this part, we are going to use the method of the previous section to determine how fast the projectile is moving as it leaves the launcher. For this part only, we shoot the projectile horizontally, so we can find this velocity.

1. Fire horizontally several times and record the distance, R, that the projectile travels (in m). Take the average.
2. Measure the height when the projectile leaves the table.
3. As we did in the previous experiment, we will use the trick of finding the time the projectile is in flight by determining how long it takes to fall. This can be simply found just knowing the height (in meters) and using the equation, , where g is 9.8 m/s². Table 8-1 in the previous section gives the time, t, for various heights.
4. Now, it is a simple matter to find the velocity using the technique of the previous section. Divide the distance the object goes along the floor, R (in meters), by the time it was in flight, t (seconds). This is given by the formula: v = R / t

This article is brought to you by: