SAT Subject Physics Practice Question 524: Answer and Explanation

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Question: 524

10. A rocket sled rides on a frictionless track while a 1000N force from the rockets accelerates the sled. Suddenly the retrorockets accidentally fire, applying a 1000N force in the opposite direction. The sled will

A. slow down gradually to a stop.
B. continue to accelerate.
C. move at a constant speed.
D. reverse direction.
E. not be affected at all.

Correct Answer: C


The correct answer is (C). The rocket sled is already moving. The two rocket engines apply two (different pairs) of forces of equal magnitude to the same object but in different directions. There is therefore no unbalanced force and the sled continues in its state of motion.

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