GCSE Physics

Energy
Forces
Forces and Motion
Waves in Matter
Electricity
Magnetism and Electromagnetism
Particle Model of Matter
Atomic Structure
Space Physics

Gravitational Forces

Gravity

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.

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

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.

Weight

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.

Remember:

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.

Quizzes