Ping Pong Catapult

Background Information for Teachers

The ping pong catapult is powered by rubber bands. When you pull back the arm of the catapult, you stretch the rubber bands, and they store potential energy. There are different types of potential energy. Many middle and high school science textbooks first deal with gravitational potential energy, or the amount of potential energy something has based on its mass and height above the ground. However, in this case you are dealing with elastic potential energy, or the potential energy materials have when stretched, bent, or compressed. The more you stretch the rubber bands, the more elastic potential energy they store.

When you release the catapult's arm, the rubber bands contract, and their potential energy is converted to kinetic energy, or the energy of motion, of the ping pong ball, which flies through the air. An object's kinetic energy depends on its mass (how heavy it is) and its velocity (how fast it is going). If two objects are moving at the same speed, the heavier one will have more kinetic energy. If two objects have the same mass but are moving at different speeds, the faster one will have more kinetic energy. Note that the ping pong ball also has gravitational potential energy depending on its height off the ground.

Conservation of energy states that energy cannot be created or destroyed; it can only change from one form to another. In this case, most (but not all) of the elastic potential energy from the rubber bands is converted to kinetic energy and gravitational potential energy of the ball. Some of the potential energy is converted to heat energy because of friction. Even though it is commonly said that this energy is "lost," it is important to remember that it has not actually vanished—it has just changed from one form to another.

The key question your students will investigate in this activity revolves around energy changing from one form to another. What happens if you stretch the rubber bands farther initially? That means they store more potential energy, and due to conservation of energy, the ping pong ball will have more kinetic energy when it launches (since its initial height will always be the same, its initial amount of gravitational potential energy will not change). Since the ball's mass remains constant, that means it will be going faster, and it should fly farther. But, do not give that answer away to your students—let them figure out an experiment to measure it!

Additional Background Links

• What is a Projectile?, the Physics Classroom
• Potential Energy, the Physics Classroom
• Kinetic Energy, the Physics Classroom

Prep Work (10 minutes)

Review this section and try using the catapult yourself before you use it with your class.

1. Review Figure 1 to familiarize yourself with the different parts of your ping pong catapult:

Image Credit: Ben Finio, Science Buddies / Science Buddies

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.

Figure 1. Parts of the ping pong catapult.

2. Unpack your catapult. During shipping, the arm is folded inside the base and they are held together by the locking pin. Remove the locking pin and unfold the arm.
3. Use the clamp to attach the base to the edge of a sturdy table or desk. The base is metal, so you may want to use a paper towel as a pad to avoid scratching furniture.
4. Insert the locking pin through the hole in the base and one of the small holes in the disc. This sets the launch angle (Figure 2), or the angle at which the ball will travel (relative to the ground) when it leaves the catapult.
5. Loop a rubber band through the large hole in the disc and hook both ends over the pin sticking through the catapult arm, as shown in Figure 2. The rubber band will pull the arm forward until it hits the stopper. When you pull the arm back, you set the pull-back angle (Figure 2), or the angle through which the arm will swing before it hits the stopper. Note that the pull-back angle is read from the top edge of the arm when you pull it back. Also note that the pull-back angle and the launch angle can be adjusted independently of each other.

Image Credit: Ben Finio, Science Buddies / Science Buddies Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 2. Pull-back angle and launch angle.

6. Load the ping pong ball into the suction cup, pull the arm back, and release. The arm should swing forward until it hits the stopper, launching the ball across the room. If you set up a tape measure or metersticks end-to-end, you can measure how far the ball travels before it hits the ground. This requires a spotter to watch where the ball lands while you operate the catapult.
7. The ping pong catapult allows you to easily adjust four independent variables:
   • the launch angle (move the locking pin to different positions on the disc)
   • the pull-back angle (adjust how far back you pull the arm)
   • the type of ball (the kit comes with a ping pong ball and a wiffle ball)
   • the number of rubber bands (the kit comes with three)
   Tinker with these variables to get a sense of the maximum range of your catapult relative to how much space is available in your classroom.
8. You should now see that you can easily set up an experiment where you change one of the four independent variables mentioned in step 7 and measure how far the ball travels as a result. The goal of this activity is to change the amount of potential energy stored in the rubber bands by changing the pull-back angle, which means you need to keep the other variables constant. But remember, you want to let your students design the experiment themselves-do not tell them how to do it! You may need to provide some guidance on good settings for the other variables; for example, if students want to use all three rubber bands and are launching balls into your classroom wall, encourage them to use fewer rubber bands.

Engage (10 minutes)

1. Throw a ping pong ball to one of your students. Encourage the students to play catch or toss the ball around the room. Explain that when the ball is moving, it has kinetic energy, the energy of motion. The faster it moves, the more kinetic energy it has.
2. Ask someone to hold the ball above their head. Even when the ball is not moving, it also has gravitational potential energy depending on how high it is off the ground (higher up means it has more energy). So, when the ball is flying through the air, it has both kinetic energy and gravitational potential energy.
3. Pass out rubber bands and ask students to stretch them. Explain that when they stretch the rubber bands, they have elastic potential energy, the type of energy materials have when they are stretched, bent, or compressed. The more the rubber bands are stretched, the more potential energy they have. Note that, while there are different types of potential energy, sometimes we just say "potential energy" and the context defines which type we are referring to. For example, in this project, when we say "potential energy" in reference to the rubber bands, we are always talking about elastic potential energy.
4. Introduce the catapult to your students (you can set it up yourself for a demonstration).

   Can anyone explain how the catapult works in terms of energy?

   Due to conservation of energy, when the rubber bands are stretched (by pulling back the catapult arm) and then released, their potential energy is converted to kinetic energy and gravitational potential energy of the ball.

5. Walk the students through the parts of the catapult and how to measure both the pull-back and launch angles. A slideshow is available to help introduce the catapult to your students as well as the video below.

https://www.youtube.com/watch?v=pIEjwMhnAGo

Explore (40 minutes)

1. Start a class discussion about what will happen if the rubber band in the catapult is given more potential energy.

   How could we change the amount of potential energy in the rubber band?

   Increasing the catapult's pull-back angle stretches the rubber band farther, increasing its potential energy.

   What will happen to the ping pong ball if we increase the amount of potential energy initially stored in the rubber band?

   If the rubber band has more potential energy, more energy will be transferred to the ping pong ball. The ping pong ball will always start out at the same height, so its gravitational potential energy will be the same. Due to conservation of energy, that means the ball will have more kinetic energy, so it will move faster, and should travel farther.

2. Remind the class that what they have now is a hypothesis: a guess, based on all of the information they have at hand, about how something works. To test a hypothesis you need to run an experiment.
3. Ask the class to brainstorm an experiment where they measure how the potential energy stored in the catapult's rubber band impacts how far the ball travels. Remember—resist the urge to tell your students how to set up the experiment; learning to design and conduct experiments is a fundamental part of learning science. If needed, ask the students questions to get them on the right track. They can use the student worksheet to help design their experimental procedure.

   Experiments often have an independent variable (something you change) and one or more dependent variables (things you measure to see if they are affected by the independent variable). What should we choose for our independent and dependent variables? Why? How will we measure them?

   The independent variable is the pull-back angle of the catapult arm, measured by the markings on the catapult's disc. The dependent variable is the horizontal distance the ball travels, measured with a tape measure or meter sticks.

4. Here is an example of a good experimental approach, which you can guide your students towards during the discussion if they get stuck.
   1. Move some desks and chairs out of the way to make a clear launching area in the classroom.
   2. Use the clamp to attach the catapult to a desk and aim it at the launching area.
   3. Set up a tape measure or end-to-end metersticks to measure how far the ball travels. Note that the "zero" mark on the tape measure should be directly under the catapult, so part of it may actually be under a desk.
   4. Come up with a data collection table. For example, students could test pull-back angles in five-degree increments and measure how far the ball travels, with three trials for each angle.
5. Have students do the experiment. Work in small groups to collect data. For example, one student could operate the catapult, two students could act as spotters to watch where the ball lands, and a fourth student could record data. If you only have one catapult for the class, groups of students can rotate through using the catapult to collect data for different pull-back angles.
6. Make a graph of data for the entire class, with horizontal distance on the y-axis and pull-back angle on the x-axis.

Reflect (10 minutes)

Ask students to reflect on the experiment and analyze the data. Here are some questions you can ask:

What are some possible sources of error in the experiment? For example, how hard was it for the spotters to measure exactly where the ball landed? Did the person operating the catapult pull the arm back to exactly the same angle for all three trials?

What is the general relationship between the pull-back angle and how far the ball travels? If you pull the catapult arm back farther, does the ball go farther? What does this mean in terms of kinetic and potential energy?

What is the shape of the relationship between the two variables? Is the graph linear (a straight line) or nonlinear (a curve)?

Assess

Use this quiz to assess student learning after the activity; quiz is available in online and pdf formats:

• Online quiz, assignable in any LMS
• Quiz (pdf) and answer key (pdf)

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Mechanical Engineer

Add Favorite Remove Favorite More Menu

• Read More

Career Profile

Mechanical engineers design many types of machines with moving parts that store energy or exert forces. They have to select what materials the machines will be built out of based on their strength and cost, and take into account how parts for the machines will be manufactured. Read more

Mechanical Engineering Technician

Add Favorite Remove Favorite More Menu

• Read More

Career Profile

A mechanical engineering technician might build a prototype of a catapult or other machine in a machine shop, make computer drawings of the machine to prepare for manufacturing, and perform laboratory tests on different versions of the machine to see how they perform. Read more

Lesson Plan Variations

• Have your students conduct experiments to measure the effects of changing the other independent variables. What happens if you change the number of rubber bands, the type of ball, or the launch angle?
• The catapult can also be used to introduce Newton's laws of motion. For example, Newton's first law states that an object in motion will stay in motion unless acted upon by some outside force. This explains why the ping pong ball keeps moving forward when the catapult arm hits the stopper. The stopper exerts a force to stop the catapult arm from moving, but there is nothing to stop the ping pong ball from continuing to move forward.
• The catapult can be used to study projectile motion. For example, a classic high school physics homework problem asks, "What launch angle will cause a projectile to go the farthest?" Be careful, however—the standard answer to this question assumes the projectile starts and lands at the same height. The answer changes if you are launching balls onto the floor from a table. See the Science Buddies project The Physics of Catapult Projectile Motion for a detailed explanation of the related math.
• The catapult can be used to introduce normal distributions and histograms. What happens if you keep all four independent variables the same and launch the ball 100 times? Does it always land in exactly the same spot?