AP Physics 1 Question 136: Answer and Explanation

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Question: 136

6. A children’s toy consists of a cart whose very light wheels are attached to a rubber band. This rubber band can wind and unwind around the axle supporting the wheels.

This toy is given a shove, after which the toy rolls across a flat surface and up a ramp. It is observed that the toy does not go a consistent distance up the ramp—in some trials it ends up higher than in other trials, even though the shove imparts the same kinetic energy to the cart each time. Which of the following is a reasonable explanation for this phenomenon?

  • A. Depending on how the rubber band is initially wound, more or less potential energy can be transferred from the rubber band to the kinetic energy of the car’s motion.
  • B. The normal force on the cart’s wheels will be different depending on how much the rubber band winds or unwinds.
  • C. How much energy is transferred from kinetic energy to gravitational potential energy depends on the vertical height at which the cart ends up.
  • D. Some of the cart’s initial kinetic energy will be dissipated due to work done by friction.

Correct Answer: A


A-Choice B is wrong-the normal force on the flat surface is equal to the cart's weight, regardless of the rubber band. Choice C is true but does not explain different heights in each trial-the problem said that the kinetic energy provided to the cart was the same every time. Choice D may or may not be true but is irrelevant in any case-even if kinetic energy is lost to work done by friction, neither the force of nor the coefficient of friction changes in different trials, so that can't explain different heights. Now, the rubber band, though, that can change things. If it's initially wound and able to unwind as the cart moves, it can transfer some of its elastic potential energy to kinetic energy of the cart. Or, if it's initially unwound, it will require some kinetic energy in order to wind up again and store elastic potential energy.