Newton's Third Law and Force Pairs

AP Physics 1· difficulty 2/5

A horse pulls on a cart. By Newton's 3rd law, the cart pulls back on the horse with <strong> equal </strong> force. Yet the system can accelerate. Why?

A
$The ground supports the horse.$
B
$These forces act on <strong> different </strong> objects, so they don't cancel within either.$
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C
$Newton's 3rd law fails for non-rigid systems.$
D
$The horse exerts more force than the cart.$

Explanation

The 3rd-law pair acts on different objects (one on horse, one on cart), so they can't cancel. The cart accelerates because the cart's net force (forward pull from horse minus friction) is forward.

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