Gravitational Forces


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

We are pulled towards the ground due to gravitational forces. These forces pull objects 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 measures 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, an object weighing 50 kg has a greater mass than one weighing 10 kg. 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 measures the gravitational force acting on an object due to 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 can be calculated if you know the object’s mass and the gravitational field strength.

Calculating Weight

Gravitational field strength varies depending on whether you are on the Moon, in space, or on Earth. 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

This means you would weigh more on Earth than on the Moon or in space.


An object’s mass remains the same regardless of its location. However, an object’s weight can change if it moves to a location with a different gravitational field strength.