Lesson 4: Newton's Third Law of Motion

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Identifying Action and Reaction Force Pairs

According to Newton's third law, for every action force there is an equal (in size) and opposite (in direction) reaction force. Forces always come in pairs - known as "action-reaction force paris." Identifying and describing action-reaction force pairs is a simple matter of identifying the two interacting objects and making two statements decribing who is pushing on who and in what direction. For example, consider the interaction between a baseball bat and a baseball.

The baseball forces the bat to the right (an action); the bat forces the ball to the left (the reaction). Note that the nouns in the sentence describing the action force switch places when describing the reaction force.

Consider the following three examples. The action force is stated; determine the reaction force. Use the "pop-up menu" to view the answer.

Athlete pushes bar upwards.

Depress mouse to view answer. Bar pushes athlete downwards.

Bowling ball pushes pin rightwards.

Depress mouse to view answer. Pin pushes bowling ball leftwards.

Balloon wall pushes compressed air inwards.

Depress mouse to view answer. Compressed air pushes balloon wall outwards.

Test Your Understanding

1. Consider the interaction depicted below between foot A, ball B, and foot C. The three objects interact simultaneously (at the same time). Identify the two pairs of action-reaction forces. Use the notation "foot A", "foot C", and "ball B" in your statements. Depress the mouse on the "pop-up menu" to view the answer.

Depress mouse to view answer. The first pair of action-reaction force pairs is: Foot A pushes ball B to the right (action); and ball B pushes foot A to the left (reaction). The second pair of action-reaction force pairs is: Foot C pushes ball B to the left (action); and ball B pushes foot C to the right (reaction). NOTE: It matters not which force of the pair is declared to be the action and which is the reaction.

2. Identify at least six pairs of action-reaction force pairs in the following diagram.

Depress mouse to view answer. The Strongman's feet push forward on the ground; the ground pushes backwards on his feet. The left end of the rope pulls forward on the Strongman's hands; his hands pull backwards on the left end of the rope. The right end of the rope pulls leftward on the monkey; the monkey pulls rightward on the right end of the rope. The left end of the second rope pulls rightward on the monkey; the monkey pulls leftward on the left end of the second rope. The tree pulls rightward on the right end of the second rope; the right end of the second rope pulls leftward on the tree. etc., etc.

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Lesson 4: Newton's Third Law of Motion

• Newton's Third Law
• Identifying Action and Reaction Force Pairs