Anyone who has ever played around on skates knows that when you push forward on the wall of a skating rink, you recoil backward.
Newton’s Third Law tells us that the force that the skater exerts on the wall,
, is exactly equal in magnitude and opposite in direction to the force that the wall exerts on the skater,
. The harder the skater pushes on the wall, the harder the wall will push back, sending the skater sliding backward.
Newton’s Third Law at Work
Here are three other examples of Newton’s Third Law at work, variations of which often pop up on SAT II Physics:
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You push down with your hand on a desk, and the desk pushes upward with a force equal in magnitude to your push.
A brick is in free fall. The brick pulls the Earth upward with the same force that the Earth pulls the brick downward. When you walk, your feet push the Earth backward. In response, the Earth pushes your feet forward, which is the force that moves you on your way. |
The second example may seem odd: the Earth doesn’t move upward when you drop a brick. But recall Newton’s Second Law: the acceleration of an object is inversely proportional to its mass (a = F/m). The Earth is about 1024 times as massive as a brick, so the brick’s downward acceleration of –9.8 m/s2 is about 1024 times as great as the Earth’s upward acceleration. The brick exerts a force on the Earth, but the effect of that force is insignificant
