The Balloon and the Balloon Car
Students build a balloon-powered car. A balloon is attached to a straw that points backward. When the balloon is inflated and released, air rushes out backward and the car rolls forward. On a smooth floor, the car travels 3 meters. On a carpet, the same car travels only 1 meter. Students also notice that a larger balloon makes the car go farther on the same floor. They want to explain the motion using forces and energy.
1. Explain why the car moves forward when air rushes out backward.
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The balloon pushes air backward. By Newton's third law, the air pushes the balloon, and therefore the car, forward with an equal and opposite force. That forward force overcomes friction and starts the car moving. The motion continues until the forward push stops and friction slows the car. So the car moves because of an action-reaction pair between the balloon and the escaping air.
2. Use energy and friction to explain the difference between the smooth floor and the carpet, and why a larger balloon helps.
Read the worked answer
A larger balloon stores more elastic energy when inflated. When released, that energy becomes motion of the air and car, giving the car more speed and travel distance. On carpet, friction is greater than on a smooth floor, so more of the car's motion energy is changed to heat and sound. That leaves less energy for rolling forward, so the car stops sooner. The carpet result is shorter because friction removes energy faster.