SAT Subject Physics Practice Question 19: Answer and Explanation

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Question: 19

10. Two crates are stacked on top of each other on a horizontal floor; crate #1 is on the bottom, and crate #2 is on the top. Both crates have the same mass. Compared with the strength of the force F1 necessary to push only crate #1 at a constant speed across the floor, the strength of the force F2 necessary to push the stack at the same constant speed across the floor is greater than F1 because

A. the force of the floor on crate #1 is greater
B. the coefficient of kinetic friction between crate #1 and the floor is greater
C. the force of kinetic friction, but not the normal force, on crate #1 is greater
D. the coefficient of static friction between crate #1 and the floor is greater
E. the weight of crate #1 is greater

Correct Answer: A

Explanation:

With crate #2 on top of crate #1, the force pushing downward on the floor is greater, so the normal force exerted by the floor on crate #1 is greater, which increases the friction force. B, C, D, and E are all false.

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