Newton’s Law of Gravitation (Gravity) (Also seen in GCSE Physics 3)
Gravity is an attractive force that acts between all masses. It is the masses themselves that cause the force to exist. The force that acts between two masses, m1 and m2, whose centres are separated by a distance of r is given by:
This was tested experimentally in a lab using large lead spheres and was refined to become:
G is the Gravitational Constant, G = 6.67 x 10-11 N m2 kg-2
When one of the masses is of planetary size, M, the force between it and a test mass, m, whose centres are separated by a distance of r is given by:
The minus sign means that the force is attractive, the force is in the opposite direction to the distance from the mass (displacement). This will become clearer when we look at the electric force.
Negative = Attractive
Positive = Repulsive
Force is measured in Newtons, N