Students will learn that Newton's 3rd law holds that for every force there is an equal but opposite reaction force. Important to note that the force and reaction force act on different objects.
Key Equations
Example 1
Question : Tom and Mary are standing on identical skateboards. Tom and Mary push off of each other and travel in opposite directions.
a) If Tom and Mary have identical masses, who travels farther?
b) If Tom has a bigger mass than Mary, who goes farther?
c) If Tom and Mary have identical masses and Tom pushes twice as hard as Mary, who goes farther?
Solution
a) Neither. Both Tom and Mary will travel the same distance. The forced applied to each person is the same (Newton's Third Law). So which cancels to Therefore both people will travel the same distance because the acceleration controls how far someone will travel and Tom and Mary have equal acceleration.
b) Mary will go farther. Again, the same force is applied to both Mary and Tom so Since Tom has the larger mass, his acceleration must be smaller (acceleration and mass are inversely proportional). Finally, because Mary's acceleration is greater, she will travel farther.
c) Neither. Newton's Third Law states that for every action there is an equal and opposite reaction. Therefore if Tom pushes twice as hard as Mary, Mary will essentially be pushing back with the same strength. They will therefore travel the same distance.
Watch this Explanation
Simulation
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Answers
Discuss in class.