**AP Physics 1 Question 61: Answer and Explanation**

### Test Information

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**Question: 61**

**1.** 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 f on the block. If the person is now pushing downward on the block with the same force 30° above the horizontal plane across the same rough surface, what is the friction on the block? (µ_{k} 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 = μ_{k}N, 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 f_{1} = μ_{k}(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 f_{2} = μ_{k}(1.5mg) = 3f_{1}.