AP Physics 1 Question 322: Answer and Explanation

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Question: 322

10. Pulling a block of mass m to the right by a connected string at an angle of 30 degrees above the horizontal (as shown in the left picture) with a force equal to the block's weight produces a friction force Fkf. If the same block were to be pulled at an angle of 30 degrees beneath the horizontal (as shown in the right picture), what would be the friction force? Assume that the applied force is enough to make the block move in both cases.

  • A. 3Fkf
  • B. 2Fkf
  • C. Fkf/2
  • D. Fkf/3

Correct Answer: A

Explanation:

A

You know Ff = µFN. In both cases, μ will be the same, so you need only concern yourself with FN. There are three vertical forces in this problem: gravity, the normal force, and the vertical component of the applied force. In the first case, that means FN = Fg - mgsinθ = mg - mgsin30 = mg - mg/2 = mg/2.

In the second case, FN = Fg + mgsinθ = mg + mgsin30° = mg + mg/2 = 3mg/2. Therefore, the force in this case will be 3 times what it was in the first case.