AP Physics 1 Question 343: Answer and Explanation

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Question: 343

1. A sphere starts from rest atop a hill with a constant angle of inclination and is allowed to roll without slipping down the hill. What force provides the torque that causes the sphere to rotate?

  • A. Static friction
  • B. Kinetic friction
  • C. The normal force of the hill on the sphere
  • D. Gravity

Correct Answer: A



Draw out a free-body diagram of all the forces acting on the sphere:

Gravity points straight down and does affect the object but it does not provide the torque (eliminate (D)). The normal force points perpendicular to the plane and does no work on the object nor does it provide the torque (eliminate (C)). The friction in this case does provide the torque. Since the ball is rolling down the hill and not sliding down it, it is not kinetic friction (eliminate (B)). This makes sense, because when you drive a car, the tires grip the road and move you forward at one point. If there were no static friction on the road, you would never be able to go anywhere because the tires would not be able to grip onto anything.