Physics VI: Newton's Laws

by Oren Lahav

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This series of lessons is designed to help you learn, or review, the fundamentals of physics. This part focuses on Newton's Laws of Forces and Motion.

Who's Newton?

Remember the guy who supposedly discovered gravity when an apple dropped on his head? That's our guy Newton. He was one of the greatest physicists and mathematicians to walk the Earth, formulating the laws that form the basis of classical mechanics.

One of the things Newton thought about was motion. Newton provided us with 3 laws of motion that make perfect sense and help us learn the effects of forces.

Law 1- Inertia

This law states that unless you activate some forces on an object that is at rest or moving at a constant rate, the object won't change its velocity. Underlying this law is inertia, the property of all objects to resist a change in motion.

This law is simple to understand. Unless you push something, it won't move. The constant speed part is a bit more conceptual, since nothing in real life has no force acting on it- anything that's moving has some friction or something else associated with it. But logically thinking, if nothing it touching or acting upon an object that's in constant motion, what would cause that object to stop? Nothing.

Law 2- Force Formula

This law says in simple terms that the rate of change of velocity is directly proportional to the force acting on the object, and inversely proportional to the object's mass.

This is super obvious- you push something light, it'll move farther than something heavy. You push it harder, it'll move even farther. See?

The formula, then, is as simple as

$a=\frac{F}{m}$ .

Note that here, F has to be the net force. This formula is useful, as we'll see next time in force problems.

Law 3- Action/Reaction

This law says this: every action has an equal and opposite reaction. This law is slightly more difficult to understand, so we'll use an example:

Imagine you're in a swimming pool. You push the pool's wall with your feet, and move forward. Now think- when you pushed the wall, you exerted a force on the wall going backwardsâ€¦ if that's all that happened, how come you moved forward? This is simple when you look at action/reaction. You applied a force on the wall, but looking at law 2, since the wall has a huge mass, it'll move very little. At the same time, there is an opposite and equal reaction- the wall exerts a force on you, in the opposite direction, forward. Since you're mass is smaller, you'll accelerate forward.

Now things make some sense, don't they? But it's important to remember that we have to look only at relevant forces. If we want to analyze the motion of a box you're pushing across the floor, we should look at the force you exert on the box, but the force the box exerts on you is irrelevant to the motion of the box.

Consider- Newton never read this stuff on LearnHub, he figure it all out on his own. What a genius!

Next time, we'll take an in-depth look at the different types of forces that are interesting to us.

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