AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Gravity: weight, W, in newtons, N
Weight is the gravitational force acting on an object, measured in newtons (N).
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 6 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Weight is the gravitational force acting on an object, measured in newtons (N). It is a vector quantity because it acts towards the centre of the planet or moon producing the field. The newton is the SI unit of force; one newton is the force needed to give a 1 kg mass an acceleration of 1 m/s². Weight is calculated using W = m g, where m is mass in kilograms and g is gravitational field strength in N/kg. For example, a 50 kg student on Earth has weight W = 50 kg × 9.8 N/kg = 490 N, but on the Moon the same student weighs about 80 N because g is smaller. Weight is measured with a newton meter or force sensor, while mass is measured with a balance. Because weight is a force, it must always be reported in newtons, never in kilograms.
What earns the marks
- Identify weight as a force measured in newtons (N) and directed towards the centre of the attracting body.
- Distinguish weight from mass, stating that mass is in kilograms and is independent of gravitational field strength.
- Use W = m g to calculate weight when mass and gravitational field strength are known.
- Describe how to measure weight using a newton meter or force sensor, and mass using a balance.
- Explain that weight changes with location because g changes, while mass remains constant.
Common mistakes and how to put them right
Saying weight is measured in kilograms.
Put it right by stating weight is a force measured in newtons.
Confusing mass and weight in calculations.
Put it right by using mass in kg in W = m g and giving the answer in N.
Forgetting that weight acts downwards towards the centre of the planet.
Put it right by describing its direction as towards the centre of the field source.
Exam tips
- Always write the unit N after a weight value and kg after a mass value to show you know the difference.
- If asked to describe how to measure weight, name a newton meter and mention reading in newtons.
- When comparing weights on different planets, state the value of g used for each location.
- Use the wording 'force of gravity acting on the object' when defining weight.
Try the lesson
The start of the Gravity: weight, W, in newtons, N lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following correctly identifies the symbol and standard unit for weight?
Listen
Weight is the gravitational force acting on an object, measured in newtons (N). It is a vector quantity because it acts towards the centre of the planet or moon producing the field. The newton is the SI unit of force; one newton is the force needed to give a 1 kg mass an acceleration of 1 m/s². Weight is calculated using W = m g, where m is mass in kilograms and g is gravitational field strength in N/kg. For example, a 50 kg student on Earth has weight W = 50 kg × 9.8 N/kg = 490 N, but on the Moon the same student weighs about 80 N because g is smaller. Weight is measured with a newton meter or force sensor, while mass is measured with a balance. Because weight is a force, it must always be reported in newtons, never in kilograms. A strong answer earns marks for these points. Identify weight as a force measured in newtons (N) and directed towards the centre of the attracting body. Distinguish weight from mass, stating that mass is in kilograms and is independent of gravitational field strength. Use W = m g to calculate weight when mass and gravitational field strength are known. Describe how to measure weight using a newton meter or force sensor, and mass using a balance. A common slip: Saying weight is measured in kilograms. Instead, stating weight is a force measured in newtons. Exam tip: Always write the unit N after a weight value and kg after a mass value to show you know the difference.
Key words
Say what each word means before you turn the card.
Weight
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Mass
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Gravitational field strength
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Newton meter
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See every definition
- Weight
- The gravitational force acting on an object, measured in newtons (N) and directed towards the centre of the attracting body.
- Mass
- Measured in kilograms with a balance, it is independent of gravitational field strength and remains constant.
- Gravitational field strength
- Given in N/kg, it is used with mass in the equation W = m g to calculate weight.
- Newton meter
- A device, along with a force sensor, used to measure weight by giving a reading in newtons.
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