Gravity

All objects with mass produce a gravitational field. The more mass an object has, the greater its gravitational field will be.

Planet size and gravitational field strength

Weight is the force acting on an object due to gravity - it has the unit newtons (N) and acts towards the centre of a gravitational field. The weight of an object can be measured using a calibrated spring-balance, often called a Newton meter.

Weight is a non-contact force because gravity exerts its force through a field. An object does not need to be touching the Earth to have a weight.

Space, the Moon landing and different planets are used to discuss gravity

Gravitational field strength (g) is measured in newtons per kilogram (N/kg). The Earth's gravitational field strength is 9.8 N/kg. This means that for each kg of mass, an object will experience 9.8 N of force.

Where there is a weaker gravitational field, the weight of an object is smaller. For example, the gravitational field strength of the Moon is 1.6 N/kg. This means that for each kg of mass, an object will experience 1.6 N of force. Therefore, an astronaut will weigh less on the Moon than they do on the Earth.

The feeling of weightlessness

Weight caused by gravity is not always obvious. Astronauts on the International Space Station appear to be weightless. However, they are still orbiting the Earth and are within its gravitational field, so they still have weight. They seem to be weightless because both they and the space station are constantly falling towards the Earth.

Professor Brian Cox explores the feeling of weightlessness that is caused by freefall