AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Changes in energy: 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, describes the gravitational force acting on each kilogram of mass at a point.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Gravitational field strength, g, describes the gravitational force acting on each kilogram of mass at a point. Its unit, N/kg, follows from weight = mass × gravitational field strength, W = m × g, so g is the weight per unit mass. Near the Earth's surface g is about 9.8 N/kg, often rounded to 10 N/kg, while the Moon's value is about 1.6 N/kg. In any calculation the value of g will be given, so read it from the question rather than assuming 9.8 or 10.
For example: A 5 kg mass on a planet with g = 4 N/kg has weight 20 N, and lifting it 2 m stores E_p = 5 × 4 × 2 = 40 J. Because g is a field strength, it acts on every kilogram equally, so doubling the mass doubles the weight but leaves g unchanged.
What earns the marks
- Define gravitational field strength as the gravitational force per unit mass, measured in newtons per kilogram.
- Use the relationship W = m × g to calculate weight when g is given.
- Interpret N/kg as equivalent to m/s² for acceleration due to gravity, without confusing the two quantities.
- Substitute the value of g supplied in the question into E_p = m × g × h or W = m × g.
- Explain that g varies with location, for example being weaker on the Moon than on the Earth.
Common mistakes and how to put them right
Assuming g is always 10 N/kg.
Put it right: Use the value stated in the question, which may be 9.8 N/kg or a value for another planet.
Writing the unit of g as N or kg.
Put it right: g is force per unit mass, so its unit is N/kg.
Confusing mass in kilograms with weight in newtons.
Put it right: Mass is the amount of matter, while weight is the force m × g.
Exam tips
- Underline the value of g given in the question and use it exactly as printed.
- Show the equation, substitution and answer with unit when calculating weight or gravitational potential energy.
- If a question compares two locations, comment on how the different values of g change weight but not mass.
Try the lesson
The start of the Changes in energy: 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
In the equation for gravitational potential energy, E_p = m g h, what does the symbol g represent and what is its standard unit?
Listen
Gravitational field strength, g, describes the gravitational force acting on each kilogram of mass at a point. Its unit, N/kg, follows from weight = mass × gravitational field strength, W = m × g, so g is the weight per unit mass. Near the Earth's surface g is about 9.8 N/kg, often rounded to 10 N/kg, while the Moon's value is about 1.6 N/kg. In any calculation the value of g will be given, so read it from the question rather than assuming 9.8 or 10. For example, a 5 kg mass on a planet with g = 4 N/kg has weight 20 N, and lifting it 2 m stores E_p = 5 × 4 × 2 = 40 J. Because g is a field strength, it acts on every kilogram equally, so doubling the mass doubles the weight but leaves g unchanged. A strong answer earns marks for these points. Define gravitational field strength as the gravitational force per unit mass, measured in newtons per kilogram. Use the relationship W = m × g to calculate weight when g is given. Interpret N/kg as equivalent to m/s² for acceleration due to gravity, without confusing the two quantities. Substitute the value of g supplied in the question into E_p = m × g × h or W = m × g. A common slip: Assuming g is always 10 N/kg. Instead, use the value stated in the question, which may be 9.8 N/kg or a value for another planet. Exam tip: Underline the value of g given in the question and use it exactly as printed.
Key words
Say what each word means before you turn the card.
Gravitational field strength
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Weight
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Mass
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N/kg
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See every definition
- Gravitational field strength
- The gravitational force acting on each kilogram of mass at a point, measured in newtons per kilogram.
- Weight
- The force calculated as mass multiplied by gravitational field strength, W = m × g.
- Mass
- The amount of matter, which stays the same when gravitational field strength changes.
- N/kg
- The unit of gravitational field strength, following from weight = mass × gravitational field strength.
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