Bombs Away! A Ping Pong Catapult
AbstractIf you were in a raiding army in the Middle Ages, a catapult would come in mighty handy for taking down castle walls. But only if you could aim it reliably! With this science project, you will try your hand at catapult technology. Using a rubber-band-powered catapult you will send ping pong balls flying through the air. The catapult's design makes it easy to measure and repeat how hard the ball is launched and its direction, so you can find the right catapult settings to hit the target reliably.
Ben Finio, Ph.D., Science Buddies
- Project based on Peloton Systems, LLC. (May, 2010). Xpult Instructions—Basic Version. Retrieved September, 27, 2012.
Recommended Project Supplies
In this applied mechanics project, students will find the right settings to reliably launch a ping-pong ball from the catapult into a target (such as a bucket or a trash can) from a certain distance away.
This fun project explores a simple machine, the catapult. In the Middle Ages these were used to hurl huge stones to break down castle walls—here you will be using a slightly smaller (and much less dangerous) version to toss ping pong balls. The catapult used for this project is easy to adjust; it also has convenient markings for reading out launch angle and "pull-back" angle (we will explain what those angles mean in the next paragraph).
The catapult kit is made up of 3 main parts which include the base, disc and arm. The base is secured to a flat surface using an included clamp. The base and arm are both connected to a rotating disc that allows the arm to swing back and forth freely. A locking pin is inserted into the base and disc so only the arm may move freely. A rubber band connects the base of the arm to the disc so when the arm is pulled back the rubber band will launch it forward. At the end of the arm a small cup holds a ping pong ball that will be launched when the catapult is completely assembled.
The base of the catapult, shown in Figure 1 above, is attached to a surface (such as the edge of a table) with a clamp. The ball (either a ping pong ball or a Wiffle® ball) sits in a cup at the end of the "launch arm," which is attached to the base. The catapult is powered by rubber bands, which connect the launch arm to the "disk." The disk makes it easy to measure and set the launch angle and the pull-back angle. The pull-back angle is how far you pull the launch arm back before you launch the ball. The launch angle is the direction the ball travels (relative to the ground) when it is launched. So, a launch angle of 90 degrees means the ball will go straight up, and a launch angle of 0 degrees means the ball will go horizontal. You set the launch angle by placing the pin through holes that line up on the disk and the base of the catapult. Figure 2 below can help you understand these angles. We will explain how to measure them in the Experimental Procedure.
When the arm of the ping pong catapult is pulled back the arc distance it travels is called the pull back angle. When the arm springs forward and launches a ping pong ball the angle it leaves the cup is the launch angle. The launch angle is measured from an imaginary line parallel to the ground at the center point of the ping pong ball just before it is launched.
You can change four things to adjust how far the ball goes:
- The type of ball (ping pong or Wiffle)
- The number of rubber bands (up to three)
- The launch angle (from 0 to 75 degrees)
- The pull-back angle (from 0 to 120 degrees)
Think about how each of these things will change how the ball is launched, and which ones you will want to adjust to make the ball hit your target.
Terms and Concepts
Before continuing with this project, make sure you understand the following terms and concepts. We have defined some of them for you in the Introduction, but you might need to look others up on your own.
- Pull-back angle
- Launch angle
- Which launch angle will cause the ball to travel farthest?
- 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?
- Ping Pong Catapult Kit is available from our partner Home Science Tools.
The kit includes:
- Catapult with easy to change settings
- Rubber bands (3, each 3 x 1/8 inch)
- Ping pong ball
- Plastic Wiffle ball
- Clamp for attaching the catapult to a surface.
- Surface for mounting catapult (piece of wood on the floor, tabletop, etc.)
- Optional: paper towels, dish towel, or other padding for protecting the mounting surface from being scratched by the attachment clamp
- Open area for launching balls
- Bucket or shoebox for catapult target
- Tape measure (12 ft. minimum)
- Lab notebook
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Safety Note: The catapult is designed to be safe, but it can hurt you if you are not careful! Never launch a ball at other people. The launch arm can move very fast (especially when using two or three rubber bands), so always make sure your fingers and other body parts are safely out of the way before you launch the ball.
- Watch this video to learn how to set up your catapult:
Set up the catapult and the target as shown in Figure 3.
- The recommended distance from the middle of the catapult disk to the middle of the target is 8 feet (ft.).
- Shoe boxes and buckets make good targets.
- The catapult clamp may scratch furniture, so you might want to use padding such as a paper towel when attaching the catapult.
- Make sure the catapult and the target are at roughly the same height.
- Insert the pin with the ring attached through the hole in the catapult base, lined up with a hole in the disk. This sets the launch angle (don't worry, you can always change this angle later). Figure 4 shows you how to do this in steps A through D.
- Attach at least one rubber band (for safety reasons, no more than three) to the pins on the launch arm by threading it through the hole on the catapult's disk. Figure 5 shows you how to do this—ask an adult if you need help.
- You are almost ready to launch! First, you will need to understand how to measure the launch angle and pull-back angle from the markings on the catapult disk. Remember that the pull-back angle is how far back you pull the launch arm before you let go and launch the ball, and the launch angle is the direction the ball is going when it is launched. Refer back to Figure 2 in the Introduction tab if you need to understand the angles. Figure 6 shows you how to measure them on the catapult.
Aim for the target, and fire away! Remember there are four things you can change: the type of ball, the number of rubber bands, the launch angle, and the pull-back angle. However, if you start changing multiple things at once, you no longer have a fair test and your results may be too confusing to interpret. So, for starters it makes sense to keep three things the same and only change one thing at a time.
- For example: you could pick the orange ping pong ball, one rubber band, and a launch angle of 75 degrees. Then you can experiment with different pull-back angles without changing anything else, and find out what pull-back angle works best to hit the target.
Go back to the Questions outlined in the Background tab, and try to use those to help you hit the target.
- For example, do you think adding more rubber bands will make the ball go farther or less far?
- Maybe you find that your ball is always falling a few feet short of the target. If so, go back and pick another variable to explore. For example, should you make the pull-back angle bigger or smaller if that happens?
See if you can find different combinations of settings that enable you to hit the target. Are some settings more reliable than others? An easy way to measure this is to take ten shots at the target with the same settings and count how many hit the target.
- Note that, depending on the material of your target, the ball may be more likely to bounce out (for example, a hard metal pot instead of a softer cardboard shoebox). But you can still count bounce-outs as a "hit."
- Also, the catapult may move or wiggle slightly if you launch the ball really hard. If you notice that you are missing your target far to the left or to the right, it is fine to adjust the clamp and re-aim your catapult.
- Write down your results in a table similar to Table 1. This shows just one example of how a data table could be used to keep track of results for a certain test.
|Number of successful shots out of 10|
|Ping pong ball, 45 degree launch angle|
|Pull-back angle||1 rubber band||2 rubber bands||3 rubber bands|
Table 1. A data table such as this could be used to keep track of the results of your experiments with settings. In this example, two things (the type of ball and the launch angle) are kept the same. Change the pull-back angle and the number of rubber band, one at a time. Then fill in each empty cell with the number of successful shots out of ten for that setting.
- Have you found some methods that are more successful at hitting the target than others? Why do you think this happens?
For troubleshooting tips, please read our FAQ: Bombs Away! A Ping Pong Catapult.
Ask an Expert
You could do many things to make this project more fun or challenging. Here are just a few ideas:
- The directions say to put the catapult and the target at the same height. What happens if you don't do this? For example, if you put the catapult on a table and the target on the floor, or the other way around? Do your results change if you try the same experiments?
- Buckets and shoe boxes are pretty big targets. Is your aim good enough to hit a much smaller target, like a coffee mug?
- What happens if you change the distance between the catapult and the target to something other than 8 ft.? How much do you have to adjust your settings to hit the target?
- What happens if you launch an object other than the plastic balls supplied with the catapult kit? For example, a small toy, or a wad of paper? Do you have to use different settings to hit the target? Safety: remember, never aim the catapult at a person, and do not launch sharp objects or things that might break—for example, anything made of glass.
Frequently Asked Questions (FAQ)
The Science Buddies catapult kit is used in four projects:
- Bombs Away! A Ping Pong Catapult
- Under Siege! Use a Catapult to Storm Castle Walls
- Launch Time: The Physics of Catapult Projectile Motion
- Bet You Can't Hit Me! The Science of Catapult Statistics
The FAQs below are used for all four projects. General catapult questions are at the top of the FAQs list and project specific questions are at the bottom of the list.
Note: On older versions of the Xpult brand catapult, the pull-back angle may be read from the bottom edge of the launch arm. If you just ordered a new Science Buddies catapult kit for your project (as of October 2012), then you should follow the directions above.
Depending on your camera, you may also be able to manually adjust settings and decrease the exposure time. Consult your camera's manual to see if that is possible.
If your theoretical and experimental results are very different, there are a couple things you can check:
- Are your theoretical predictions reasonable? You should be able to launch the ball across an average-sized room in your house — meaning it should go a couple meters before it hits the ground, for typical catapult settings. If your predictions indicate that the ball will travel several kilometers, or only a couple millimeters, then odds are you made a mathematical mistake. Double-check your calculations, and be extra careful to look for typos if you entered equations into a spreadsheet program.
- Make sure you properly used a scale factor to convert the distances you measure on your computer screen to real-world distances for your experimental data. Again, make sure the distance the ball travels is reasonable — if your experimental data says that the ball traveled 100 meters, then you probably used the scale factor incorrectly.
Four example histograms display the same data with bin sizes of 1, 5, 10 and 20. As the bin sizes change, the distribution of data becomes too compact or spread out to determine whether or not the data is normally distributed from a glance. Bin sizes of 5 and 10 show a clearly normal distribution of data, but bin sizes of 1 and 20 are less obvious.
Notice how the shape of the distribution is evident with bin sizes of 5 or 10, but rather difficult to see with bin sizes of 1 or 20. As a general rule of thumb, more data will allow you to use a smaller bin size, which will give you a more accurate picture of the distribution — so if you have time, try to do more than 50 trials with the catapult.
For the person recording distances, we recommend watching where the ball lands and immediately marking the location with your finger. Then take a reading from the tape measure. Trying to watch where the ball lands and simultaneously read the tape measure will be very difficult.
Also, you probably won't be able to get fraction-of-an-inch accuracy on your readings — getting the closest one-inch increment will be good enough.
If you like this project, you might enjoy exploring these related careers:
Contact UsIf you have purchased a kit for this project from Science Buddies, we are pleased to answer any question not addressed by the FAQ above.
In your email, please follow these instructions:
- What is your Science Buddies kit order number?
- Please describe how you need help as thoroughly as possible:
Good Question I'm trying to do Experimental Procedure step #5, "Scrape the insulation from the wire. . ." How do I know when I've scraped enough?
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