What is Newton’s Third Law?

Newton’s Third Law of Motion is my personal favorite. Why? Because it serves a dual purpose as good life advice. It goes like this.

When you apply a force, expect an equal amount of force to be applied back.

It also just so happens that Newton’s Third Law is also one of the most fun to demonstrate and seriously makes you question virtually everything in the universe.

Imagine yourself pushing into a wall. You can even get up and give it a shot. Now, odds are that the wall didn’t move. I’m willing to bet that you didn’t move either. Now imagine you are pushing into a wall with both hands, but you’re standing on a skateboard.

What happens now?

You start to roll back. Why?

Newton’s Third Law.

It explains what launches rockets, propels squids, keeps you grounded, and just about everything in between.

The best way to think about Newton’s Third Law is in opposites, and we call them action and reaction forces.

Your cat jumps off of a table and lands on the counter. The action force? Pushing off of the table. The reaction force? The table pushing back up at the cat.

Remember when we talked about support force and equilibrium? The main example was how when you stand on an old-school spring scale, it is pushing back up at you. That’s how a scale tells you your weight. We said, actually, the whole world is like that. The ground is always pushing upwards against you to keep equilibrium with the force of your weight. A book being pushed down on a table is always being pushed back up.

Equilibrium is key in this universe. It means balance. We know from Newton’s First Law that moving things keep moving unless stops it and not moving things keep not moving unless something moves them, right?

When we say the first law, we use the term unbalanced forces. That’s the trick. Unbalanced forces cause changes. Balanced forces, forces that are equal and opposite, either keep things still or keep things moving. Balanced forces are always two or more, happening at the same time.

That’s as good a spot as any to transition to this next bit, and a good question.

If Newton’s Third Law says that for every action, or force, their is an equal and opposite one, how does anything happen? Like, if I’m pushing a cart, and the cart is pushing me back, how come it doesn’t just sit there?

The answer lies in net force. Remember that net force is the total sum of forces.

As I start to push the cart, some of that overall force either coming from me or to me gets redirected. Some will be the applied force of my push, some will be the friction of the cart as it moves across the floor.

It’s important to remember one thing. Words all have meanings, right? Some words are words for other words. The word force is just another word that we use to describe interactions between two or more objects, things, people, plants, planes, or whatever else we might be describing.

Next time, we start exploring some real-world examples of Newton’s Laws in action!