Gravitational forces

People often confuse mass and weight. Remember that weight is a force that acts upon a mass, and is measured in newtons, N. Mass is measured in kilograms, kg.

Mass

The mass of an object is the amount of matter or ‘stuff’ it contains. The more matter an object contains, the greater its mass. An elephant contains more matter than a mouse, so it has a greater mass.

Mass is measured in kilograms, kg. A 100 kg object has a greater mass than a 5 kg object. An object's mass stays the same wherever it is. So a 5 kg mass on Earth has a 5 kg mass on the Moon.

The force of gravity

Gravity is a force that attracts objects towards each other. Gravity 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.

Characters standing at various points around the Earth, showing how gravity always pulls us towards the centre‘Down’ is towards the centre of the Earth, wherever you are on the planet

Tom Kerss, astronomer at the Royal Observatory Greenwich, talks about gravitational forces

Weight

The weight of an object is the gravitational force between the object and the Earth. The weight of an object depends upon its mass and the gravitational field strength.

Gravitational field strength is given the symbol g. Do not confuse this with g for grams. You can use this equation to calculate the weight of an object:

curriculum-key-fact
weight in N = mass in kg × gravitational field strength in N/kg

On Earth, g is about 10 N/kg. This means that a 2 kg object on the Earth’s surface has a weight of 20 N (2 kg × 10 N/kg = 20 N).

Mass and weight

The mass of an object stays the same wherever it is, but its weight can change. This happens if the object goes where the gravitational field strength is different from the gravitational field strength on Earth, such as into space or another planet.

The Moon is smaller and has less mass than the Earth, so its gravitational field strength is only about one-sixth of the Earth’s. So, for example, a 120 kg astronaut weighs 1200 N on Earth but only 200 N on the Moon. Remember that their mass would still be 120 kg.

On Earth, an astronaut has a mass of 120 kg. His weight is 120 x 10 = 1200 N. On the moon his mass is still 120 kg but his weight is only 200 N.A 120 kg astronaut weighs less on the Moon than they do on Earth

Tom Kerss, astronomer at the Royal Observatory Greenwich, on why humans weigh less on the Moon

Jon Chase explains that the mass of an object is constant but its weight depends on the gravitational force