Every push has a push back
How a balloon rocket works, why you slide backwards when you push a wall on ice, and Newton’s third law.
Let go of a blown-up balloon and it zooms around the room. It seems to move all by itself. But something is pushing it. What?
The balloon rocket
When you let go, the air rushes out of the back of the balloon. The balloon pushes the air backward. And at the same time, the air pushes the balloon forward.
That is Newton's third law of motion:
When one thing pushes on another, the second thing pushes back on the first with the same size of force, in the opposite direction.
People often say: "Every action has an equal and opposite reaction."
It is true for every push
Push on a wall. The wall pushes back on your hand with the same force. You can feel it. If you pushed on a wall while wearing roller skates, the wall would not move, but you would roll backwards.
Here are two skaters on ice. When one pushes the other, both move:
Each push comes with a push back. That is why both slide away from each other.
More examples
- Jumping. You push down on the ground. The ground pushes you up.
- Swimming. You push the water backward. The water pushes you forward.
- Walking. Your foot pushes the ground backward. The ground pushes you forward.
- A gun's recoil. The gun pushes a bullet forward, and the bullet pushes the gun back.
Rockets
Rockets work the same way as the balloon. The engine throws burning gas out of the bottom, very fast. The gas pushes the rocket up.
A common mistake is to think rockets need air to push against. They do not! In space, a rocket still pushes gas out of its back and the gas still pushes the rocket forward. This is why rockets work in empty space.
In Orbit Keeper, you launch a satellite at 7.7 km/s. In real life, every bit of that speed is given by a rocket pushing gas the other way.
Why don't the two forces cancel?
This puzzles many people. If every push has an equal push back, why does anything move at all?
The answer is that the two forces act on different things. When the balloon pushes the air, the force acts on the air. When the air pushes back, the force acts on the balloon. To cancel, forces must both act on the same object. Here, they do not, so the balloon accelerates.
Who moves more?
Back to the skaters. Suppose a light skater pushes a heavy skater. The forces are equal. But remember: acceleration is force ÷ mass. The light skater has a small mass, so she speeds off quickly. The heavy skater has a big mass, so he barely moves.
Thread a long string through a straw and tie it between two chairs. Tape a blown-up balloon (pinched shut) to the straw. Let go! The balloon shoots along the string. Try a bigger balloon, and see how far it goes.
Words to know
- Action and reaction
- The pair of forces when two things push on each other.
- Newton's third law
- Every push has an equal push back in the opposite direction.
- Thrust
- The force that pushes a rocket or jet forward.
- Recoil
- The backward push you feel when something is pushed away from you.
Check yourself
When you push on a wall, what does the wall do?
Show the answer
It pushes back on you with the same size of force.
How does a balloon rocket move?
Show the answer
It pushes air backward, and the air pushes the balloon forward.
Can a rocket work in empty space?
Show the answer
Yes. It pushes gas out the back and the gas pushes the rocket forward. It does not need air to push on.