Bomb's Away! A Ping Pong Catapult

Related Links

• Science Fair Project Guide

Project Summary

Difficulty	4  –  5
Time required	Very Short (a day or less)
Prerequisites	None
Material Availability	Specialty items
Cost	Low ($20–$50)
Safety	Minor injury possible

Sponsor

Sponsored by a generous grant from Seagate

Objective

This project uses a rubber-band powered catapult called the Xpult. The goal of the project is to find launch conditions to reliably propel the payload (a small plastic ball) into the target area (a box or bucket placed a fixed distance from the catapult).

Introduction

This is a fun project for exploring the mechanics of a simple machine, the catapult. The catapult used for this project is easy to adjust; it also has convenient markings for reading out launch angle and "pull-back" angle.

Figure 1. The Xpult catapult.

The Xpult catapult is pictured in Figure 1, above. It's a simple, rubber band-powered device. The base of the catapult is clamped down to a solid surface, as shown. The ball to be launched (ping pong ball or golf 'whiffle' ball) is held in a cup at the end of the catapult arm. One or more rubber bands are attached to pins on either side of the arm. The rubber bands are threaded through a hole in the aluminum disk, as shown. The disk can be rotated, in 15° increments, to set the launch angle. A pin on the right side, below the catapult base, locks the disk in place at the desired launch angle. The left side of the disk also has angle markings, so that the "pull-back angle" can be measured. This angle measures how much tension is placed on the rubber bands before launch.

You have four independent variables that you can you adjust in order to get the payload to the target:

1. type of ball,
2. number of rubber bands,
3. launch angle,
4. pull-back angle.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• launch angle,
• pull-back angle,
• trajectory.

Questions

• For the same settings (number of rubber bands, launch angle and pullback angle) which ball do you think will travel further, the ping pong ball or the ball with holes? Why?
• Which launch angle will cause the ball to travel furthest?
• If you increase the pull-back angle, what will happen to the launch distance?
• If you increase the number of rubber bands, what will happen to the launch distance?

Bibliography

• The Xpult catapult kit is available from:
  Peloton Systems, LLC, 2004. "Xpult Experimental Catapult," Peloton Systems LLC [accessed March 21, 2006] http://www.xpult.com/index.htm.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• Xpult catapult kit (available from http://www.xpult.com/index.htm). The kit includes:
  • 3 rubber bands (size: 3 inches × 1/8 inch),
  • table tennis ball,
  • plastic ball with holes,
  • clamp for attaching the catapult to a surface.
• surface for mounting catapult (piece of wood on the floor, tabletop, etc.),
• open area for launching balls,
• bucket or shoebox for catapult target,
• tape measure, (12 ft minimum).

Experimental Procedure

Safety Note: The Xpult is designed to be safe. However, it does store and release energy and therefore could cause injury. Never launch a ball at other people. Pay particular attention to the end of the launch lever and make sure your eyes and other body parts are clear before releasing it.

1. Set up the catapult and target as shown schematically in Figure 2, below.
   1. The suggested distance between the catapult and target is 96 inches, measured from the center of the catapult disk to the center of the target.
   2. A 5-gallon bucket or a shoebox make good targets.
   3. The edges of the catapult may scratch furniture, so you might want to use padding underneath the catapult base before clamping it into place.

   Figure 2. Setting up the catapult. Measure distance from the center of the catapult disk to the center of the target box or bucket.

2. Measuring launch angle and pull-back angle. Figure 3, below, shows a close-up view of the Xpult catapult.

   Figure 3. Measuring catapult launch variables.

   1. The ends of the rubber bands attach to pins on the launch arm of the catapult. The rubber bands pass through the hole in the aluminum disk. For safety reasons, use no more than 3 rubber bands.
   2. The launch angle is set by placing the locking pin underneath the catapult base through one of the launch angle holes. These are marked in 15° increments on the right side of the aluminum disk. Read the launch angle underneath the catapult base (as indicated by the corresponding red line in Figure 3, above).
   3. The pull-back angle is measured on the left side of the aluminum disk, at the back edge of the launch arm (as indicated by the corresponding red line in Figure 3, above). Note that you can set the pull-back angle to any value; you're not limited to the discrete values indicated by the tick marks.
3. Choose a ball and you're ready to start launching.
   1. As you work towards the goal of hitting the target, think about the questions above. Formulate a hypothesis about how to get closer to the target, and test your hypothesis.
   2. By all means, investigate multiple variables, but change only one variable at a time.
4. Can you find multiple sets of variables for hitting the target reliably?
   1. A simple way to measure how reliable the settings are is to take 10 shots with the same settings and count how many of the 10 are on-target.
   2. Are some sets more reliable than others? Why?

Variations

• The catapult can also be used to investigate variability. Even with the same settings on the catapult, the ball will not travel exactly the same distance on every trial. Think of possible reasons for this. Use repeated trials to plot a frequency histogram for distance traveled. Are the results normally distributed? How many trials do you have to carry out to get a reliable estimate of the distribution? Do different settings have more, less or the same degree of variability? Why?

• For more science project ideas in this area of science, see Mechanical Engineering Project Ideas

Credits

Seagate Technology

Edited by Andrew Olson, Ph.D., Science Buddies

Sources

• Ulrich, K. and C. Terwiesch, 2002. "Xpult Instructions," The Wharton School, Department of Operations and Information Management, University of Pennsylvania [accessed March 21, 2006] http://www.xpult.com/Xpult%20(designed%20and%20distributed%20by%20Terwiesch%20and%20Ulrich).pdf.

Last edit date: 2006-04-20 00:14:59