Forces - 5.1.3 Gravity (GCSE Physics AQA)

Gravity

Weight and Gravity

The Link Between Weight and Gravity

• Weight is the force on an object due to gravity. Without gravity, objects would just have mass. With gravity, objects have a force pulling them down due to their mass.
• The gravitational field is responsible for the force of gravity close to Earth. Around the earth, there is a gravitational field which pulls objects down. Anything with mass experiences this gravitational pull, including objects (e.g. a chair) and living things (e.g. humans).

Calculating Weight

Formula for Weight

We can calculate weight using a simple formula:

Where:

• Weight (W) is in newtons (N)
• Mass (m) is in kilograms (kg)
• Gravitational field strength (g) in newtons per kilogram, N/kg.

Gravitational Field Strength

• Gravitational field strength varies throughout the Solar System. Different planets can have their own gravitational field, and the strength of the field can vary. For example, on the Moon, the gravitational field strength is lower than on the Earth.
• Gravitational field strength determines the weight of an object. We can use the strength of the gravitational field to help calculate the size of the gravitational ‘pull’ an object experiences, and hence its weight. For example, the same astronaut will weigh less on the Moon than on the Earth. This is simply due to the lower gravitational field strength on the Moon.

Weight and Mass

Differences between Weight and Mass

• There is a difference between mass and weight. Weight is determined by the action of gravity, whereas mass is not. This means that wherever you are in the Solar System, your mass will be the same, but your weight will not.
• Weight is a force. Weight is measured in newtons and changes according to the force of gravity. This means that your weight on the Earth is different to your weight on the Moon.
• Mass is just a value. Mass is the ‘amount’ of object that you have. Your mass on Earth would be the same as your mass on the Moon.

Weight and Mass are Directly Proportionals

• Weight and mass are directly proportional. Weight and mass are closely linked, and are said to be directly proportional. This means that they do the ‘same thing’. If the mass halves, then the weight will half. If the mass triples, then the weight will triple.

Measuring Weight

Centre of Mass

• The centre of mass will be at the centre of a symmetrical object. The centre of mass is the point at which the object will balance. It is often found in the centre of a symmetrical object.

• The weight of an object acts at the centre of mass. Weight will act as a uniform force, directly downwards from the centre of mass.

Using a Newtonmeter

• Newtonmeters are used to measure weight. Newtonmeters are calibrated spring-balances, which are used to measure weight. When an object is placed onto the end of the newtonmeter, the spring will extend. Then the force (weight) can be read off the scale.