The force that pulls objects downwards towards the centre of the earth is called gravity or gravitational force. When we throw a ball up, we throw it with a lot of force—more than the force of gravity. So it rises up. As the ball moves up, it loses this force and then gravity pulls it down. Newton realised that gravity was an important kind of force in explaining the way the universe works. The value of g does vary in different places on the Earth due to factors such as the Earth's rotation, and the density of the rock under our feet.This means that every 1 kg of mass causes a weight (on Earth) of 9 . 81 N . We can usually approximate this to 10 N .The letter g is called the gravitational acceleration and its value depends on the size of the masses causing the attraction. On Earth, gravitational acceleration is affected by the huge size of the Earth's mass. This means that the value of g is more or less the same wherever we are on Earth: g = 9 . 81 m s - 2 .