**Gravitational Force**

Newton's Law of Universal Gravitation describes how ALL objects attract each other's mass with a force of gravitational attraction.

The force of gravitational attraction was found experimentally to be **directly proportional** to the masses of each of the objects and **inversely proportional** to the square of the distance that separates their centers.

To make this into an equation we need to add a 'constant of proportionality'. This is 'G' - the **gravitational constant**. The precise value of G was determined experimentally by Henry Cavendish in 1798, almost a century after Newton's death. It has a value of 6.673 x 10^{-11} N m^{2} kg^{-2}, and you will find it on your data sheet in examinations. It is also known as the **universal gravitational constant**, **Newton's constant**, and colloquially as **'Big G**'.

In the AQA A level syllabus the distance between the masses is denoted by 'r'.

Mass m_{1} is pulling mass m_{2}, and mass m_{2} is pulling mass m_{1}... newton's third law!

**Click here** to find out about 'g' - the gravitational field strength

**Click here** to find out about gravitational potential