University of Edinburgh

BSL Physics Glossary - Newton's 3rd Law - definition

You saw me pushing this wall. I can't push it over. I'm pushing one way, but the wall is exerting a force the other way. These two forces are balanced. They are equal but work in opposite directions. As I push the wall, the wall exerts an opposite force on me.

A similar thing happens with me standing here on the floor. My feet are pressing down on the floor. The floor is pushing up on my feet. They are balanced forces.

When I am in a swimming pool, I push my arms backwards, and at the same time I move forwards through the water. There are opposite forces balancing each other. 

Newton explained this 3rd law about opposite forces.

When a rocket is ready to be launched it is exerting a force down into the ground. What happens when the engines start? We'll look at this model of a rocket. It's a balloon. It's similar to a rocket. It's got air in it. What happens if I let the air out? Let's have a look. Ready?

The air blasted out of the back of our rocket balloon. And another force moved in the opposite direction to move the balloon forward. The forces acted in opposite directions. I think it went round and round at the same time because the mouth of the balloon was bent. If the air had come straight out the back, the balloon would have gone straight ahead. We can see the same with a real rocket. The rocket engines push downwards, and the rocket moves upwards. This is an example of Newton's third law.

Would you like to make a rocket balloon at home? This is what you need: some string, a drinking straw, a balloon (long ones are better. Mine was round), a peg and some sellotape. 

First thread the straw onto the string, and fix the string up across the room. Here's one I made earlier. Take your balloon and blow it up. Oh no!
Luckily I've got another one. Don't tie the balloon up, peg it. The air can't get out. Now sellotape the balloon to the straw at the top.

Now are you ready? One, two three, go! It went fast. You can try this at home. Ask your parents for help. Bye!