Irving, a cat, has a favorite toy that is made up of a squeaky ball at the end of a string. The toy makes up a closed system. Irving’s owner dangles the toy so that the ball swings down and then swings back up again. Ignoring any friction, how are kinetic and potential energy related to each other as the ball swings back and forth?
Answer
A
As the ball moves upward, kinetic energy decreases and potential energy increases.
B
As the ball moves downward, both kinetic and potential energy increase.
C
As the ball moves upward, both kinetic and potential energy decrease.
D
As the ball moves downward, kinetic energy decreases and potential energy increases.
Explanation
As the ball swings upward, it gains height, so its gravitational potential energy increases. Because friction is ignored in this closed system, total mechanical energy stays constant; the increase in potential energy comes from a decrease in kinetic energy. Therefore, choice A is correct.
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