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 pairs." Identifying and describing action-reaction force pairs is a
simple matter of identifying the two interacting objects and making two
statements describing who is pushing on whom and in
what direction. For example, consider the interaction between a baseball bat
and a baseball.
The baseball forces the bat to the left; the bat forces the
ball to the right. Together, these two forces exerted upon two different
objects form the action-reaction force pair. Note that in the description of
the two forces, the nouns in the sentence describing the forces simply switch
places.
Consider the following three examples. One of the forces in
the mutual interaction is described; describe the other force in the
action-reaction force pair. Click the button to view the answer.
Baseball pushes glove leftwards.
The glove pushes
the baseball rightward.
Bowling ball pushes pin leftwards.
Pin pushes
bowling ball rightward.
Enclosed air particles push balloon wall outwards.
Balloon wall
pushes enclosed air particles inwards.
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Check 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. Click the button to view
the answer.
The first pair of
action-reaction force pairs is: foot A pushes
ball B to the right; and ball B pushes foot A to the left. The second pair of
action-reaction force pairs is: foot C pushes ball B to the left; and ball B
pushes foot C to the right.
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2. Identify at least six pairs of action-reaction force pairs
in the following diagram.
The elephant's
feet push backward on the ground; the ground pushes forward on its feet. The
right end of the right rope pulls leftward on the elephant's body; its body
pulls rightward on the right end of the right rope. The left end of the right
rope pulls rightward on the man; the man pulls leftward on the left end of the
right rope. The right end of the left rope pulls leftward on the man; the man
pulls rightward on the right end of the left rope. The tractor pulls leftward
on the left end of the left rope; the left end of the left rope pulls rightward
on the tractor. etc., etc.