AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Gravitational field strength, g, in newtons per kilogram, N/kg (In any…
Full specification wording
Gravity: gravitational field strength, g, in newtons per kilogram, N/kg (In any calculation the value of the gravitational field strength (g) will be given.)
Gravitational field strength, g, is the force in newtons acting on each kilogram of mass, so its unit is newtons per kilogram (N/kg or N kg⁻¹).
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 8 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Gravitational field strength, g, is the force in newtons acting on each kilogram of mass, so its unit is newtons per kilogram (N/kg or N kg⁻¹). On Earth g is about 9.8 N/kg, on the Moon about 1.6 N/kg, so the same 10 kg object weighs about 98 N on Earth but only 16 N on the Moon. In the equation W = m × g, g tells you the weight per unit mass. In any calculation the value of g will be given in the question, so read it carefully and use the stated value rather than assuming 9.8 N/kg.
For example: With m = 5 kg and g = 1.6 N/kg, W = 5 × 1.6 = 8 N. Always write the unit N/kg with the value and check that the weight answer is in newtons.
What earns the marks
- State that gravitational field strength is measured in newtons per kilogram (N/kg).
- Explain that g represents the weight in newtons acting on each kilogram of mass.
- Use the value of g supplied in the question when substituting into W = m × g.
- Recognise that g differs between planets and moons, so weight changes while mass stays constant.
- Calculate weight by multiplying mass in kg by the given g and give the answer in newtons.
Common mistakes and how to put them right
Assuming g is always 9.8 N/kg when the question supplies a different value.
Put it right by using the value printed in the question.
Writing the unit as Nkg or N/kg⁻¹ incorrectly.
Put it right: The accepted forms are N/kg or N kg⁻¹.
Multiplying mass by g but giving the answer in kg or N/kg.
Put it right by stating the weight in newtons.
Exam tips
- Highlight the value of g given in the question and copy it exactly into your working.
- Show the equation W = m × g, then substitute the numbers, so the method is visible.
- Check the unit of your final answer: weight must be in newtons, not kilograms.
Try the lesson
The start of the Gravity: gravitational field strength, g, in newtons per kilogram, N/kg (In any calculation the value of the gravitational field strength (g) will be given.) lesson, made from checked teaching notes for this specification point.
Have a go first
A pupil is told that the gravitational field strength on the Moon is 1.6 N/kg. What does the unit N/kg mean?
Listen
Gravitational field strength, g, is the force in newtons acting on each kilogram of mass, so its unit is newtons per kilogram (N/kg or N kg⁻¹). On Earth g is about 9.8 N/kg, on the Moon about 1.6 N/kg, so the same 10 kg object weighs about 98 N on Earth but only 16 N on the Moon. In the equation W = m × g, g tells you the weight per unit mass. In any calculation the value of g will be given in the question, so read it carefully and use the stated value rather than assuming 9.8 N/kg. For example, with m = 5 kg and g = 1.6 N/kg, W = 5 × 1.6 = 8 N. Always write the unit N/kg with the value and check that the weight answer is in newtons. A strong answer earns marks for these points. State that gravitational field strength is measured in newtons per kilogram (N/kg). Explain that g represents the weight in newtons acting on each kilogram of mass. Use the value of g supplied in the question when substituting into W = m × g. Recognise that g differs between planets and moons, so weight changes while mass stays constant. A common slip: Assuming g is always 9.8 N/kg when the question supplies a different value. Instead, using the value printed in the question. Exam tip: Highlight the value of g given in the question and copy it exactly into your working.
See it
Key words
Say what each word means before you turn the card.
Gravity and weight
Try answering aloud or in your head before turning the card.
Units and measurement
Try answering aloud or in your head before turning the card.
Proportional reasoning
Try answering aloud or in your head before turning the card.
Planetary comparisons
Try answering aloud or in your head before turning the card.
See every definition
- Gravity and weight
- Gravitational field strength, g, is the force in newtons acting on each kilogram of mass.
- Units and measurement
- Its unit is newtons per kilogram, written as N/kg or N kg⁻¹.
- Proportional reasoning
- In the equation W = m × g, g tells you the weight per unit mass.
- Planetary comparisons
- g differs between planets and moons, so weight changes while mass stays constant.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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