AP Physics 1 Question 338: Answer and Explanation

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Question: 338

6. A person is pulling a block of mass m with a force equal to its weight directed 30° above the horizontal plane across a rough surface, generating a friction f1 on the block. If the person is now pushing downward on the block with the same force 30° below the horizontal plane across the same rough surface, what is the friction on the block? (μk is the coefficient of kinetic friction across the surface.)

  • A. f
  • B. 1.5f
  • C. 2f
  • D. 3f

Correct Answer: D

Explanation:

D

The friction f on the block is represented by the formula f = where µkN, where N is the normal force on the block. When the force is applied 30° above the horizontal, N = mg - mg sin 30°. Since sin 30° is 0.5, N = mg - 0.5mg = 0.5mg. Substituting N into the formula for friction, it becomes f1 = μk(0.5mg). When the force is applied 30° below the horizontal, N = mg + mg sin 30° = mg + 0.5mg = 1.5mg. Substituting N into the formula for friction, it becomes f2 = μk(1.5mg) = 3f1.