Questions: Newton's Third Law: Action-Reaction Pairs

Question 1

A horse pulls a cart forward with force F. By Newton's third law, the cart pulls the horse backward with force F. A student argues this means the cart can never accelerate forward. What is wrong with this reasoning?

• A. The cart's pull on the horse is slightly smaller due to friction losses
• B. Newton's third law only applies to objects at rest, not accelerating ones
• C. The two equal-and-opposite forces act on different objects (horse and cart), so each object's motion is governed only by the net force on that object alone
• D. The horse's force on the cart is slightly larger, creating a net force forward

Answer: (C)

Action-reaction pairs always act on different objects. The horse pulls the cart forward with force F — this force acts on the cart. The cart pulls the horse backward with force F — this force acts on the horse. Apply Newton's second law to each separately: the cart accelerates forward if F exceeds friction on the cart. The paired forces never cancel because they are on different objects.

Question 2

A book sits still on a table. The force of gravity pulls it downward, and the normal force from the table pushes it upward. Are these two forces a Newton's third law action-reaction pair?

• A. Yes — they are equal, opposite, and in contact with each other
• B. No — they act on the same object (the book); Newton's third law pairs always act on two different objects
• C. Yes — any two equal and opposite forces constitute a Newton's third law pair
• D. No — action-reaction pairs must involve the same type of force, and gravity and normal force are different types

Answer: (B)

Balanced forces and Newton's third law pairs are distinct. Gravity and the normal force both act on the book — they are balanced forces on a single object producing zero net force. The actual Newton's third law partner of 'Earth's gravity pulls the book downward' is 'the book's gravity pulls Earth upward.' The third-law pair must involve the same type of force and act on two different objects.

Question 3

Newton's third law force pairs always act on two different objects.

• T. True
• F. False

Answer: True

This is the defining property that distinguishes Newton's third law pairs from balanced forces. When A exerts a force on B, B exerts an equal and opposite force on A — always on A and B separately, never both on the same object. This is why the two forces do not cancel: cancellation would require them to act on the same object.

Question 4

If you push a wall and the wall pushes back on you with an equal and opposite force, the two forces cancel and you experience zero net force.

• T. True
• F. False

Answer: False

The two forces act on different objects — your force acts on the wall, and the wall's force acts on you. 'Canceling' requires forces to act on the same object. You do experience the wall's force (which is why you don't accelerate into the wall), but that force acts only on you. The wall simultaneously experiences only your force. Net force on each object is computed from forces on that object alone.

Question 5

Why don't action-reaction force pairs cancel each other out, even though they are always equal in magnitude and opposite in direction?

Model answer: Because the two forces in an action-reaction pair act on different objects. Cancellation requires two forces to act on the same object and sum to zero. Newton's second law applies to a single object — you sum only the forces on that object. The paired force acts on the other object entirely and plays no role in the first object's equation of motion.

A book rests on a table because gravity and the normal force (both on the book) cancel — these are not a third-law pair. The third-law partner of the table's normal force on the book is the book's normal force on the table — a different force on a different object. Only forces on the same object can cancel.