Consider a bicycle that has a miniature generator that is turned by one of the b

Question

Consider a bicycle that has a miniature generator that is turned by one of the bicycle's wheels. The generator is connected to a headlight. For each part, choose one answer only.

(a) When the headlight is off, the generator

•transfers energy from the light to the bike
•transfers no energy between the light and the bike    
•transfers energy from the bike to the light
•could be any of these

(b) When the headlight is on, the generator

•transfers energy from the light to the bike
•transfers no energy between the light and the bike    
•transfers energy from the bike to the light
•could be any of these

(c) Suppose the bicycle is coasting (the rider is not pedaling) along level ground. If the rider turns on the headlight, the bicycle will

•not coast as far
•coast farther
•coast the same distance as it would have with the light off
•could be any of these

Explanation / Answer

Hi,

This problem can be solved if we imagine a system formed by the bicycle and the headlight. According to the problem, part of the energy that the driver uses to move the bicycle will be spent to turn on a the headlight. If this is true, then the generator and by extension the headlight only work when the bicycle is in movement.

(a) When the headlight is off, the generator:

If the headlight is off, either the bicycle is not moving or the generator is not working. The generator can only transfer energy from the bicycle to the headlight, assuming that the bike is moving, the right answer in this case should be:

transfers no energy between the light and the bike.

(b) When the headlight is on, the generator:

If the headlight is on, then the generator is doing its work. Again, the generator can only transfer energy from the bicycle to the headlight, so the right answer in this case should be:

transfers energy from the bike to the light.

(c) Suppose the bicycle is coasting (the rider is not pedaling) along level ground. If the rider turns on the headlight, the bicycle will:

This system, the bike and the headlight, has a limited amount of energy that can be used to move the bike or turn on the headlight. If the rider is pedaling, then he is constantly giving energy to the system, so both actions can occur at the same time. However, if that stops, and the rider dicides to turn on the light, then the distance travelled by the bicycle should be shorter (because its kinetic energy is spent in the generator). So the answer should be:

not coast as far

I hope it helps.

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