What is Newton’s Third Law of Motion?

Newton's Third Law of Motion

Isaac Newton came up with three laws of motion. We’re going to take a look at the third law, which says that every action has an equal and opposite reaction. If I were to go up to this whiteboard and push on it really hard, what’s happening is that white board is actually exerting an equal force against me.

The reason that is is because if the whiteboard wasn’t exerting an equal force upon me, then I’d be able to push the whiteboard- be able to push this whole wall over. But I’m not able to, because the wall is strong enough to be able to push back against me.

Kind of a visual way to represent this is that force 1 equals negative force 2. Notice the arrows here. They’re drawn against each other. The force I’m feeling against me is the resistance I’m feeling in my arms as I push against this whiteboard.

You could think of it visually as if I was on rollerblades and I went and pushed against this whiteboard really hard and I shoved it. I would roll backwards. The reason would roll backwards is because of this third law. It’s giving me the same force, the same push, that I’m giving it.

If I push really hard, then it’s going to push back against me really hard and I’m going to go back a long way. If I just barely push it, then it’s not going to push me back as hard. You could think about this as the recoil of a gun.

If a gun is shot, then all of the force that is given to project the bullet out the front is making the person who is holding that gun move backwards. That’s another example of this third law of motion. Another great use of this, another great example is a rocket.

I’ll draw the fire coming out of the bottom. Things are shot out of the bottom of the rocket. There is a force going down this way, we’ll call that F1, which results in this rocket moving upwards, which we’ll call -F2. These forces right here are equal to each other except that this force is just in the opposite direction.

As all that force is being pushed out it causes the rocket or the space shuttle to project upwards. And so that’s Newton’s third law of motion. Every action has an equal and opposite reaction.


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by Mometrix Test Preparation | This Page Last Updated: June 29, 2022