Lesson 4: Newton's Third Law
Identifying Action and Reaction
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 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
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.
Bowling ball pushes pin rightwards.
Balloon wall pushes compressed air inwards.
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
2. Identify at least six pairs of action-reaction
force pairs in the following diagram.