Average Force of Impact in a Tennis Ball Collision

Goal Determine the average force of impact on a tennis ball colliding with a wall.

Introduction

A tennis ball was shot horizontally from a slingshot toward a wall. In the photo to the right, NCSSM alumni Eric Deren and Sean McGrew load the slingshot. Here's a video clip of the slingshot in action. This was a segment in an ESPN documentary about NCSSM.

Now download this video clip taken with a high-speed camera at 2000 frames per second (1/2000 s per frame). Play the clip and then step through the frames one-by-one by dragging the slider. You can see the Frame # advance by 1 per frame.

Theory

Use this simple theory to guide your method: In a collision, the change in momentum of an object is equal to the impulse on that object, , where and  is the average net force acting on the object during the collision.

Method, Data, and Analysis

We're leaving it up to you to make decisions about how to measure the things that you need to calculate Fave.

• Describe your method in sentence form. Equations and numbers are necessary but not sufficient. Tell what you measured and how you measured it.

• Avoid using general terms like distance and velocity in isolation. You'll be measuring more than one distance and calculating more than one velocity, so you have to distinguish between them. Provide a legend in which you define all the symbols that you use.

• Momentum is a vector. Set up a position axis.

• You'll need a way to scale your measurements on the photo to the real world. You can use the diameter of a tennis ball for that. You'll also need the mass of a typical tennis ball. We're leaving it up to you to measure those values or look them up.

Discussion

Comment on whether your result for the average force of impact makes sense. If it helps you to think about force in pounds rather newtons, you can find a conversion factor in your textbook. Describe another way that you could estimate the force of impact. Here's a hint:  About how far is the ball squashed in the collision?