A boy pushes a shopping cart forward with 50\text{N}. By Newton's third law, the cart exerts a force on the boy of

Answer: 50\text{N} backward Newton's 3rd law pairs are equal in magnitude and opposite in direction: the cart pushes the boy backward with 50\text{N}.

A book rests on a table. Which pair are Newton's 3rd-law pairs?

Answer: Book pushing table down; table pushing book up. A 3rd-law pair acts between the same two objects in opposite directions: book↔table contact forces are the pair.

A rocket accelerates by ejecting hot gases. The reaction force on the rocket is exerted by

Answer: The ejected gases. The rocket pushes gases backward; by Newton's 3rd law the gases push the rocket forward. (Rockets work in vacuum — no air needed.)

AP Physics 1 · Topic 2.3

Newton's Third Law and Force Pairs Practice

Part of Force and Translational Dynamics.(TOP-2.C)

Practice questions

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Sample questions

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Sample 1difficulty 1/5
$50~\text{N}$
- A
  $0~\text{N}$
- B
  $50~\text{N}$
- C
  $50~\text{N}$
  check_circle
- D
  $50~\text{N}$
Why
$50~\text{N}$
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Sample 2difficulty 2/5
$A book rests on a table. Which pair are Newton's 3rd-law pairs?$
- A
  $Earth pulling book down; air pushing book up.$
- B
  $Earth pulling book down; table pushing book up.$
- C
  $Book's weight; book's mass.$
- D
  $Book pushing table down; table pushing book up.$
  check_circle
Why
$A 3rd-law pair acts between the same two objects in opposite directions: book\leftrightarrowtable contact forces are the pair.$
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Sample 3difficulty 2/5
$A rocket accelerates by ejecting hot gases. The reaction force on the rocket is exerted by$
- A
  $The atmosphere it pushes against.$
- B
  $Earth's gravity.$
- C
  $The ejected gases.$
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- D
  $Itself.$
Why
$The rocket pushes gases backward; by Newton's 3rd law the gases push the rocket forward. (Rockets work in vacuum — no air needed.)$
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Sample 4difficulty 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
  $The horse exerts more force than the cart.$
Why
$The 3rd-law pair acts on different objects (one on horse, one on cart), so they can't cancel. The cart accelerates because the cart's net force (forward pull from horse minus friction) is forward.$
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