The force of gravity
Gravity is a force of attraction that exists between all objects that have mass.
Gravity provides the force needed for:
- planets, asteroids and comets to orbit the Sun;
- moons to orbit planets;
- artificial satellites to orbit planets;
- a dropped mobile phone to fall to the ground.
Back in 1687, Sir Isaac Newton realised that the size of the force of gravity depends on two things:
- the mass of the two objects and
- the distance between the two objects.
He found that:
- the bigger the mass, the larger the force of gravity;
- the greater the distance between the two objects, the smaller the force of gravity between them.
The force of gravity acts between all masses but only becomes noticeable when there is a really massive object like a moon, planet or star.
We are pulled down towards the ground because of gravity.
The gravitational force pulls in the direction towards the centre of any object.
So, we are pulled towards the centre of the Earth.
As an astronaut travels upwards from Earth into space, the force of gravity due to the Earth decreases, because the distance between the Earth and the astronaut increases.
This is why an astronaut is weightless in space.