Newton’s Third Law

6 10 2011

One might assume that a discussion of Newton’s three laws should start with the first. When I started writing these posts I tried that. I started with half of Newton’s first law and in the discussion I found that I could only really explain Newton’s First Law if you knew the other two laws already. Then thinking about it, I realized that really I should start with Newton’s Third Law.

You’re going to have to trust me on this and just roll with it. Ok?

So Newton’s Third Law is most commonly stated as:

“For every action there is an equal and opposite reaction.”

More clearly worded, but less catchy, this clarifies to:

“The forces of two bodies on each other are always equal and are directed in opposite directions.”

It’s important to remember the second clarifying sentence, because otherwise, things might get confused when we’re discussing the other laws later on. But I’ll help you through those when they come up. For now, what you need to know is the first catchy one.

Sometimes this law is a little hard to grasp. After all, when I push against a wall, is it pushing back on me? Is that what we’re saying? If that’s true, why don’t I fall over when I push against the wall?

But let’s not talk about walls and people. Let’s talk about two kids. Essentially this means that if two kids sitting in chairs with wheels, push against each other (directly, in a line, let’s not add weird angles to this), they will both be moved by the force of the push. Yes, one kid might go further than the other, but that’s related to the fact that the two kids don’t weigh exactly the same (and that’s related to the conservation of momentum, which we’re not discussing yet). So if two kids, weighing exactly the same amount, are sitting in chairs with wheels–like the kind you use for your computer–and they press their hands against each other and both push forward, against the other, both kids will be pushed back the same distance.

Let’s clarify this yet further. Even if only ONE of the kids had pushed (meaning the other kid sat there completely stiff), BOTH kids would still have moved an equal distance back.

This is because even though the kid not moving doesn’t actively use his muscles to push on the other kid, the fact that the other kid is pushing on him, means he’s exerting an equal and opposite force.

But let’s go back to a harder to grasp example, the example of something very small (like a human) and something very big (like the earth).

You, as a small human, are exerting your weight on the surface of the earth. Gravity is pulling down on you, trying to pull you into the very earth itself, trying to pull you closer and closer to its center. (This force of gravity on you, is your weight. Aka, the more you weigh, the greater the force of gravity is on you).

So if gravity is acting on you so consistently why don’t you get pulled through the surface of the earth?

Well, because of Newton’s third law!

You are exerting a force on the ground–the force of the earth wanting to suck you in. But the ground is exerting an EQUAL and OPPOSITE force, counteracting gravity.

The equal and opposite is very important. It has to be opposite in order to directly counter act gravity. Otherwise, if the ground was exerting a force on you at an angle, you’d feel it and sort of slide sideways, because gravity isn’t counteracting that business. Gravity acts straight down, not at an angle. The equal is important, because if the force of the ground was less than gravity, you would be pulled through the ground. If the force was greater than gravity, you would actually be pushed up and away from the earth.

Because remember, a force is a push or pull. So if two people are pulling on something equally, it’s not going to move. Like a tug of war rope, when the sides are evenly matched.

So the earth and ground are both pushing on you with the same force, so you don’t go anywhere.

And by the way, we call that upwards force the ground exerts on you a Normal Force.

I hope you’re not confused, and that this adequately explained the third law.

Let me know if you have questions!