Physics

Fox

### Summary

- An object's
**net force**is the force acting on it in**one direction**,*minus*the force acting on it in the**opposite direction**.

To understand the laws from the first page of this chapter, we need to know about **speed**, **force** and **net force**. We now know about speed and force, so all that's left to understand is *net force*. Let's do that now...

You might have heard of the phrase ‘net worth’: a person's *net worth* is how much money they *own*, **minus** how much money they *owe* other people.

For example, let's say you own £200, but you owe your friend £50. Your *net worth* = money owned - money owed = £200 - £50 = £150.

Net worth is important because it's the amount of money *that you actually have*. It doesn't matter if you *own* £1 million if you also *owe* someone else £1 million! Your net worth = £0, so it's *as if* you have no money at all.

**Net force** is a similar idea to net worth: an object's *net force* is the force acting on it in one direction, **minus** the force acting on it in the opposite direction.

For example, let's imagine the forces on a car. Let's say there's a 200N force forwards, and a 50N force backwards. The car's *net force* = force forwards - force backwards = 200N - 50N = 150N.

Net force is important because it's *what actually makes things move*. It doesn't matter if you have 1,000,000N one way if there's also 1,000,000N the other way! The net force = 0, so it's *as if* there's no force acting on it at all. You might see this in a tug-of-war; the rope won't move if there's an equal pull on either side — even if both pulls are very strong!

This is the idea behind **balanced forces**. We'll learn more about these on the next page…

Note that force might be going upwards (e.g. for a balloon).

The process here is the same as above; to find the net force up, we find the force up — force down.

So the net force (upwards) = 2,100,00N - 2,000,000N = 100,000N.

Or consider a falling football; to find the net force *downwards*, we find the force down — force up.

So the net force (downwards) = 5N - 2N = 3N.

But what if there are forces both left-and-right, and up-and-down? We saw an example of this with a car on the previous page:

We have to *separately* find the net force in each direction. For example, the net force *right* = 5000N - 2000N = 3000N. The net force *upwards* = 10,000N - 10,000N = 0N.

So all 4 forces are equivalent to just 1 force to the right.

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