Physics Is Life
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Unit 2, lesson 8. About 15 minutes.

Newton's Third Law

How does a rocket push itself along when there is nothing in space to push on?

Play

Getting the experiment ready…

Explain

When you push, it pushes you back.

Rockets push gas down to go up.

Forces always come in pairs. If you push a wall, the wall pushes you back just as hard.

Two skaters who push each other both move, even if only one of them does the pushing.

Make one skater heavier. The forces stay equal, but the lighter skater moves faster.

Newton's third law: when object A exerts a force on object B, B exerts an equal and opposite force on A.

The two forces act on different objects, so they never cancel each other out. Each object responds with its own acceleration, a = F / m.

Worked example: 150 N acts on each skater for 1 s. The 20 kg skater gets 150 / 20 = 7.5 m/s, and the 80 kg skater gets 150 / 80 = 1.9 m/s.

  1. You push the wall
  2. The wall pushes back
  3. The forces are equal
Want the full story?

A force pair always has the same type of force on two different objects. Your weight and the floor pushing up on you are not a third-law pair, since both act on you. The pair to your weight is your gravity pulling the whole Earth up towards you.

The Formula

the force on one thing = the same force, the opposite way, on the other

FA on B=−FB on AF_{A \text{ on } B} = -F_{B \text{ on } A}
FA on BF_{A \text{ on } B}
the force A puts on B, measured in newtons (N)
FB on AF_{B \text{ on } A}
the force B puts on A, measured in newtons (N)

Try the numbers

Force of B on A (minus means the opposite way): -150 N

Try it at home

Ask a grown-up to help

You need a balloon, a smooth table or floor.

You need: a balloon, a smooth table or floor.

Blow up a balloon and let go. Whoosh!

  1. Blow up a balloon and pinch the neck shut.
  2. Hold it over a clear space.
  3. Let go and watch it fly.

What's happening: the balloon pushes air backward, and the air pushes the balloon forward with an equal force.

Balloons are a choking hazard. Keep burst pieces away from small children.

Physics is life

Your body

Your feet push the ground, and it pushes you.

To walk, you push back on the ground. The ground pushes you forward with the same force.

Daily life

Swimmers push the water back.

Swimmers push water backward, and the water pushes them forward.

Nature

Birds push the air down to fly up.

A bird pushes air down with its wings, and the air pushes the bird up.

Space

Rockets push hot gas down.

Rockets throw hot gas out of the back at great speed. The gas pushes the rocket forward, even in empty space.

Quick check

Question 1 of 3

What does Newton's third law say?

Words to know

Force pair: Two equal and opposite forces that two objects put on each other.

Force pair
Two equal and opposite forces that two objects put on each other.
Thrust
The push that drives something forward, like a rocket or a swimmer.
Force
A push or a pull on something.

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Next up

Friction: the force that lets us walk