Gravitational Forces


Gravity is a force that attracts objects to each other. It keeps the Moon orbiting around the Earth and it keeps the Earth in orbit around the Sun.

We are pulled down towards the ground due to gravitational forces. These forces pull us in the direction of the centre of the Earth.

Gravitational field strength is the force per unit of mass, measured in newtons per kilogram (N/kg).

  • The symbol g is used to represent gravitational field strength


Mass is a measure of the amount of matter in an object. This means that the more matter an object contains, the greater its mass is.

  • Mass is measured in kilograms (kg)

For example, a 50 kg object has a greater mass than a 10 kg object. An object’s mass stays the same no matter where it is. This means that a 50 kg object will be 50 kg on Earth and 50 kg in space.


In physics, weight does NOT mean your mass in kilograms. Weight is a measure of the gravitational force between an object and the Earth. This force is due to the pull of gravity, which acts towards the centre of the planet.

Weight is measured in newtons (N) and it can be calculated when you know the mass and gravitational field strength.

Calculating Weight

To determine the weight of an object, you need to know its mass and the gravitational field strength. The formula that we use is:

Weight (N) = mass (kg) × gravitational field strength (N/kg)

  • Weight is measured in newtons (N)
  • Mass is measured in kilograms (kg)
  • Gravitational field strength is measured in newtons per kilogram (N/kg)

Gravitational field strength is different on the Moon, in space and on Earth.

  • Earth’s gravitational field strength is 10 N/kg
  • The Moon’s gravitational field strength is 1.6 N/kg
  • The gravitational field strength in space is 0 N/kg

Which means you will weigh more on Earth compared to on the Moon and in space.


The mass of an object is the same no matter where it is. However, an object’s weight can change if it enters somewhere with different gravitational field strength.