Every Push Has a Partner

Lesson 7 of 11 · 7 min · Free, no account needed

What you will learn

Newton's third law: when A pushes or pulls B, B pushes or pulls A just as hard, in the opposite direction.
The two forces of a pair act on different bodies, so they never cancel each other.
Equal forces can still make different motions: the lighter body gets the bigger change in speed.

You push off a wall on a skateboard and roll away. You pushed the wall. So who pushed you?
Drag the skater to the wall and let go to push.
Watch both arrows at the moment of the push.
Two skaters push each other apart. Change the left skater's mass and push again.
Forces come in pairs. This is Newton's third law.
Newton's third law: if A pushes B, then B pushes A with a force of the same size in the opposite direction.
The two forces of a pair act on different bodies. That is why they do not cancel: each one moves its own body.
Simplified model: no friction under the wheels. With friction the skater still feels the wall's push; they just slow down sooner.
Three scenes. In each, tap the partner of the yellow force.
Game: space rescue. Your tether broke. Get back to the station before your oxygen runs out.
Throwing a tool pushes you the other way. Is one tool enough?
Math: equal pushes, unequal masses. Which pair makes the light cart roll 3 times as far in the same time?
A foot kicks a ball. Draw the force on the ball and the force on the foot.
A 50 kg skater and a 75 kg skater push off. The light one rolls away at 3 m/s. How fast does the heavy one go?

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