AP Physics 1 Question 158: Answer and Explanation

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Question: 158

8. A horse is attached to a cart that is at rest behind it. Which force, or combination of forces, explains how the horse-cart system can accelerate from rest?

  • A. The forward static friction force of the ground on the horse is greater than any friction forces acting backward on the cart, providing a forward acceleration.
  • B. The forward force of the horse on the cart is greater than the backward force of the cart on the horse, providing a forward acceleration.
  • C. The force of the horse’s muscles on the rest of the horse-cart system provides the necessary acceleration.
  • D. The upward normal force of the ground on the horse is greater than the horse’s weight, providing an upward acceleration.

Correct Answer: A


A-To evaluate the acceleration of the horse-cart system, you can only consider the forces applied by objects external to the system. This eliminates choices B and C, which discuss forces of the system on itself. Choice D is ridiculous because the horse remains on the ground. Choice A is correct: the ground pushes forward on the horse's hooves because the horse's hooves push backward on the ground.