# Gravitational Potential Energy

When an object is above the Earth’s surface, energy is stored in the gravitational potential energy store. This is due to the force of gravity. The gravitational potential energy of an object is determined by three factors:

• Its mass
• The gravitational field strength of the location
• Its height above a reference point, usually the ground.

Gravitational field strength

Height above the ground

The higher the object is, the more energy is stored as gravitational potential energy. When looking at gravitational potential energy, it’s important to understand mass and gravitational field strength.

## Mass

Mass is a measure of the amount of matter in an object. This means 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.

## Gravitational Field Strength

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

Gravitational forces pull us down to the ground. These forces pull us in the direction of the centre of the Earth.

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.

Gravitational field strength varies between the Earth, the Moon and outer space.

• 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

## Calculating Gravitational Potential Energy

To calculate gravitational potential energy, use the equation:

Gravitational potential energy = Mass × Gravitational field strength × Height

• Ep = Gravitational potential energy in Joules (J)
• m = Mass in kilograms (kg)
• g = Gravitational field strength in newtons per kilogram (N/kg)
• h = Height in metres (m)

When objects fall, energy transfers from gravitational potential energy to kinetic energy.

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