NEWTON'S THIRD LAW: ACTION AND REACTION
4 marks · NormalThe diagram below shows a person (A) in a small rowboat pushing another person (B) in a second rowboat. Both boats are initially at rest.
Describe what happens to both A and B when A pushes B's boat away. Refer to the forces involved.
Explain how this demonstrates Newton’s third law. Name the action and reaction forces.
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(a) Both boats move away from each other in opposite directions because they each experience a force (A to left, B to right).
(b) The force A exerts on B’s boat (action) is equal and opposite to the force B’s boat exerts back on A (reaction), demonstrating Newton’s third law.
Must show: refers to both persons moving in opposite directions, action and reaction described
Common mistakes: Describing only B moving; Forgetting reaction force on A
Show mark scheme and worked solution
Marking points
B1 (a) · 2 marks · Describes movement of both A and B’s boats in opposite directions, both experiencing a force.
B2 (b) · 1 mark · Correctly identifies the action force as A pushing on B’s boat.
B3 (b) · 1 mark · States the reaction force is B’s boat pushing back on A and links this to Newton’s third law.
Worked solution
a) When A pushes B’s boat, both boats move apart—A’s boat moves in one direction and B’s in the other. Each experiences a force in opposite directions. b) This demonstrates Newton’s third law: the action force is A pushing on B’s boat; the reaction is B’s boat pushing equally on A in the opposite direction. Both forces are equal in size and opposite in direction.