Rubber Bands for Energy

Abstract

Objective

In this experiment you will investigate how the distance of stretch in a rubber band at rest relates to the distance the rubber band travels after being released.

Credits

Sara Agee, Ph.D., Science Buddies

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Introduction

No mechanical contraption would be any fun to use if it did not work. But to do "work" in the classical sense, takes energy. Consider a rope and pulley that bring a bucket up a well. They would not work at all if there was not a person using their own energy to pull up the rope. The person in this mechanical system is providing an energy input called an applied force.

Mechanical systems need energy to do work, but where does that energy come from? To understand this you need to know that there are two different kinds of energy:

• Potential Energy (PE)- energy that is stored
  
• Kinetic Energy (KE) - energy in motion

A great example of the difference between kinetic and potential energy is from the classic "snake-in-a-can" prank. This is an old joke where you give someone a can of peanuts and tell them to open it, but inside is actually a long spring that jumps out of the can when they open it. Since the spring is usually decorated to look like a long snake, this prank usually causes the victim to jump back and shout! When the snake is secured inside the unopened can, it has potential energy. But when the can is opened, the potential energy is quickly converted to kinetic energy as the snake jumps out of the can!

The "snake-in-a-can" joke is an example of Potential Energy (PE) and Kinetic Energy (KE). (Image adapted from www.supercoolstuff.com)

In this experiment you will investigate how kinetic and potential energy work in a very simple system: a rubber band shooter. In this system you will stretch a rubber band over the end of a ruler and release it (without aiming it at anyone of course). By stretching the rubber band back to different lengths, you can describe the relationship between the amount of potential energy and the amount of kinetic energy in this system. How will they affect the distance your rubber band travels?

Terms and Concepts

To do this type of experiment you should know what the following terms mean. Have an adult help you search the Internet, or take you to your local library to find out more!

• mechanical system
• work
• energy input
• potential energy
• kinetic energy
• conversion

• If I stretch it really far, how far will the rubber band go?
• If I don't stretch it very far, how far will the rubber band go?
• Which way creates the most potential energy?

Bibliography

• At this site you can learn about the energy of motion:
  Rader, A., 2007. "Energy of Motion," Physics4Kids.com [accessed February 10, 2007] http://www.physics4kids.com/files/motion_energy.html
• Read about the different forms of potential energy:
  KEEP, 2002. "What is Energy?" Wisconsin K-12 Energy Education Program (KEEP). [accessed February 10, 2007] http://www.uwsp.edu/cnr/WCEE/keep/Mod1/Whatis/energyforms.htm
• This site will explain how all of that energy you read about does work:
  Nice, K., 2007. "How Force, Power, Torque and Energy Work," Howstuffworks.com [accessed February 10, 2007] http://science.howstuffworks.com/fpte.htm

Materials and Equipment

• metric ruler
• rubber bands (all of the same size and kind)
• metric tape measure
• helper
• sidewalk chalk

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Experimental Procedure

1. Find a helper, gather your supplies and go outside to do this experiment. You will want a place with a lot of clearance that has a cement or hard-caped surface that you can draw on with chalk. Your partner will draw circles around where the flying rubber bands land, so choose a partner with a keen eye and some running shoes!
2. Remember to bring a data table and pencil with you so that you can log your results. Here is a sample data table that would work well for this experiment:

   Distance (cm)	10cm	20cm	30cm	40cm	etc.
   Trial #1
   Trial #2
   Trial #3
   Trial #4
   Trial #5
   Trial #6
   Trial #7
   Trial #8
   Trial #9
   Trial #10
   Average Distance (cm)

3. You stand on one side of the space, and have your partner stand on the other side.
4. With your piece of chalk draw a line in front of your toes. This is where you will line your feet up when you shoot your rubber bands. This is also where you will begin measuring the distances your rubber bands have gone.
5. Choose a few different lengths along your ruler where you will pull your rubber band back to. They should be at regular intervals along the ruler. In my example above I chose 10 cm, 20 cm, 30 cm, etc. Write your values in your data table.
6. Now you are ready to conduct the experiment, starting with your first length. Shoot a rubber band by hooking it on the front edge of the ruler, then pulling back to your first length (10 cm) on the ruler and then letting go. Remember the angle and height you hold the ruler, because you will need to keep it the same for each rubber band launch.
7. Have your partner draw a circle where that rubber band landed.
8. Measure the distance from your line to the spot your partner just marked in your data table.
9. You will launch rubber bands from each length 10 times, so that you can average the data to get better results. This means that you will need to repeat steps 6-8 nine more times before moving on to the next length in your data table. Then you will repeat the procedure 10 times for each of the other lengths you have chosen.
10. Make a graph for your results by placing a scale of stretch length in cm on the bottom (x-axis) and a scale of launch distance (cm) on the left side (y-axis). Plot a dot for each data point. Do the dots follow any type of pattern? Does it look like a line or a blob? If it looks like a line, draw a line of best fit through your data.
11. Analyze your data and make your conclusions. Did your data show any trend? What was the relationship between the amount of stretch and the launch distance? What do you think this means about the relationship between potential and kinetic energy?

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Variations

• You can do a very similar experiment with these types of mechanical systems:
  • springs
  • sling shots
  • any other ideas?
• In this experiment you kept the angle and height of launch the same from trial to trial. How would these variables affect the distance the rubber band would travel? Design a separate experiment to test each of these variables separately.
• Advanced students can use linear regression to further analyze the data. Can you define an equation that expresses the relationship between potential and kinetic energy in this system? What is the error in your experiment? Are your results significant?

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Related Links

• Science Fair Project Guide
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• Mechanical Engineering Project Ideas
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