## What is Gravity?

Gravity is a force of attraction only between bodies that have mass. The word 'gravity' comes from the Latin word "gravitas", meaning 'weight'. The force of gravity that one body exerts on another can be expressed as:

(1)

F = Force of gravity experienced by bodies. m1 = mass of body.
G = Gravitational constant , 6.6726 * 10-11 m3 kg-1 s-2.  m2 = mass of second body.
r = radius of body.

From this an important equation for finding the force of gravity on an object e.g. planet.
Newton's second law states that:

``` F = m * a.             (2)
```

If you substitute the 'F' in equation 1 with 'm * a' in equation 2, you will get:

(3)

Divide both sides by m and you will get:

(4)

At a 90 degree angle:

```a = g        (5)
```
```So the new formula is:
```
`(6)`
```Knowing the gravitational constant, the mass of the Earth,
and your distance from the centre of the Earth, you can
work out the gravity of Earth.

Please note: Gravity is measured in Newtons(N) or
kg ms-1. ```
`Find out about Surface Gravity here!`
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