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From: Infinite Potential Mission 1: Critical Current (pp: 15)
Changes in Potential

In this activity, you will analyze balloon rockets to determine the relationship between potential and kinetic energy transformations.
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Changes in Potential

Solar Flares
Have you ever wound up a rubber band toy, then released it? The stored energy of the twisted elastic transforms into the energy of the toy’s motion. In a similar way, the sun’s magnetic field stores energy in its twists and turns. Part of this twisted magnetic field can lift off in a burst called a coronal mass ejection, which Janet Green studies.

In this activity, you will analyze balloon rockets to determine the relationship between potential and kinetic energy transformations. As in a twisted magnetic field, a stretched balloon can store elastic potential energy when inflated. We can observe the effects of this energy as it is transformed into mechanical kinetic energy when the balloon is released. Using the formula KE = ½mv2 we can calculate its kinetic energy. From this value, we can infer the relative amount of elastic PE stored in the stretched rubber of the balloon.

Materials
  • straws
  • stopwatch
  • meter stick
  • safety goggles
  • tape

Caution

Caution!
Review safety precautions with your instructor before beginning this lab. When working with projectiles, always wear goggles.


Lab Prep
  1. Work in a large space such as a gym or multipurpose room.
  2. Obtain a length of kite string about 10 m long. Pass the string through a drinking straw.
  3. Tie each end of the string to chairs positioned at opposite sides of the room.  Make sure the string is level and taut.  Slide the straw to one end of the string.
  4. Now inflate the balloon, but do not tie it. Use a strip of tape to attach the inflated balloon to the straw. Make sure that the nozzle points in the opposite direction ofthe intended path. Secure the balloon so that it hangs below the string.

Make Observations
  1. Release the balloon and observe its behavior. Use the stop watch to time how long the balloon rocket travels. Measure the distance traveled in meters. Record both values.
  2. Using the math tools, calculate average speed. This value can be used to represent average velocity, as average velocity is the average speed with direction.
  3. Obtain the mass of the balloon/straw system.   Discuss the order of operations and use the math tools to determine the kinetic energy of the balloon.
  4. If we assume no energy loss through transformation of elastic potential energy to kinetic energy, how much elastic potential energy was stored in the stretched rubber of the balloon? Explain.
  5. Identify factors that influenced the potential energy and kinetic energy inyour investigation.   Explain using the data you collected.
  6. Design an investigation that tests the influence of one of the factors you have identified.

Extension

Design an investigation that measures gravitational potential energy and kinetic energy of two or more objects that have been dropped.

Journal Question

Journal Question
Imagine and draw a machine that could be powered by elastic potential energy.
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