Newton's First Law

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Newton's First Law of Motion at times seems intuitive, and at other times counterintuitive. So we'll introduce the law, define a couple terms, and then we'll give examples that will help explain it.

Here's the law: "An object at rest stays at rest and an object in motion stays in motion with the same velocity unless acted upon by an unbalanced force."

Here are the definitions you'll need to understand before we dive into the law:

Velocity: Some people think of speed and velocity as the same thing, but in physics, they really aren't. Speed is simply a measure of how fast you're traveling (for example, 35 mph is a speed). Velocity is more than just how fast you're going; it's a vector (which means it has both magnitude and direction). So to fully describe someone's velocity, you might say "35 mph at 30 degrees north of east."

So when Newton says "with the same velocity," he's really saying that the object won't change either its speed or its direction.

Force: A force is a push or pull (how do you like how scientific that is?). Forces are measured in the metric unit Newton (I bet you can guess who that was named for!). A higher value means a stronger push.

Unbalanced Force: Suppose two people are pushing on a car, and they are pushing with exactly the same amount of force, but they are pushing inopposite directions. You know what those forces are? They are "balanced." They cancel each other out, and the car doesn't go anywhere. So unbalanced force is what happens when one or more forces don't cancel each other out. So if you have just one force, or if you have two or more forces that don't cancel each other out, that's an unbalanced force.

In other words

So we can state Newton's first law in a slightly different way:

If something is standing still, it'll stay standing still unless something pushes it.
If something is moving, you have to give it a push to change the way it's moving.

Is it really true?

"Wait a minute," you might argue, "when I'm riding my bike, if I stop pedaling, shouldn't the bike keep going at the same speed? It doesn't work! The bike slows down, and eventually stops!"

Good point. It almost seems like Newton's law doesn't work, because objects don't keep moving--eventually they slow down and stop! So if Newton's law is true, we have to conclude that there must be an unbalanced force acting on your bike. Right?

Well, it turns out that there is an unbalanced force. It's called friction. Friction is a force you can't get away from entirely in the real world. Air friction (or air resistance), the grinding of the gears together, the bumpiness of the road--all result in unbalanced forces that oppose the motion of your bike and slow it down.

So let's switch gears (so to speak) and move ourselves from the road to an ice rink. Imagine that you and a friend are at a skating rink. You have ice skates on, but your friend has boots that have sandpaper on the bottom of them. Someone comes up behind you both and gives you the same amount of push. Who will go further? You will, of course! The reason is that there is very little friction between your skates and the ice.

It's easy, then, to imagine that if we could do away with friction completely, you could skate forever!

The closest we can get to that ideal is probably a spaceship traveling past the earth's atmosphere. Even though there are particles in space, it's mostly vacuum, so there isn't much to produce an unbalanced force on the spaceship. So once you apply force (burning fuel) to get it up to speed, it'll take a very long time to slow back down again!