Gravitational Forces


Gravity is an invisible force that attracts objects towards each other. This becomes very noticeable when dealing with large bodies of mass, such as planets. This is because the greater the object’s mass is, the greater the gravitational force. Gravitational forces pull inwards, towards the object’s centre. This is why on Earth, we are pulled towards the ground.

A simplified representation of the Earth and the Moon. The Earth is shown in blue with green continents, and around it are three smaller illustrations: a tree with a red house behind it, the Leaning Tower of Pisa leaning. Circling the Earth is an orbit path on which the Moon, represented by a yellow circle labelled "Moon", is travelling, indicated by an arrow showing its direction around the Earth. The overall theme playfully incorporates elements from folklore and real-world landmarks to illustrate gravitational influence.

Gravitational field strength is the force per unit of mass, which is 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 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 and in space, when compared to Earth.

  • Earth’s gravitational field strength is 10 N/kg
  • The Moon’s gravitational field strength is 1.6 N/kg
  • In deep space, far from any significant masses, the gravitational field strength is approximately 0 N/kg

This 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 a different gravitational field strength.