# Gravity

#### Back to Different Types of Forces

This is a simple tutorial describing the principles of gravity, aimed at Grade 9/10 students (GCSE).

## What is gravity?

There are forces between objects that attract them to each other. These forces are referred to as *gravitational forces*. You'll likely have heard of gravitational forces before, in the form of *gravity* - it's what keeps the planets orbiting around the sun, and us from floating around in the sky.

## Why don't all objects experience a gravitational pull to each other?

Well they do; gravitational forces are not limited to large objects. In fact, *every single object experiences a gravitational pull to the objects around it *- even to you. You're probably wondering then why we don't see objects randomly moving around to get closer to each other. This is because the *magnitude (size)* of the attractive gravitational force between objects depends on a couple of factors:

- the masses of the objects involved, and
- the distance between their centers.

We can describe this relationship using the below formula, where m1=mass of the first object, m2=mass of the second object, r=the distance between their two centers, and G=the *gravitational constant*.

## What does this formula tell us?

It tells us two things:

- It tells us that the larger the objects are, the greater the pull will be.
- If you double the mass of an object, you will double the strength of the force pulling them towards each other.

- It also tells us that the closer the objects are to each other, the stronger the pull will be.
- If you halve the distance between objects' centres, you will increase the strength of the force four-fold (because 2 squared = 4).

- If you halve the distance between objects' centres, you will increase the strength of the force four-fold (because 2 squared = 4).

## Wait, so why don't everyday objects get pulled to each other?

Everyday objects *do* experience a gravitational pull towards each other, but the pull is so small that it is well outweighed by other forces, including:

- the gravitational pull of the earth (because the mass of the earth is soooo big compared to any small object on it)
- friction (e.g. on solid surfaces or even caused by air-resistance).

If these forces were removed, then objects would move closer to each other - but alas, we're likely never to see that happen.