Gravity and Weight
Master Gravity and Weight for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Weight is the force of gravity on a mass, calculated using W = mg, and acts from the object's centre of mass.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Weight is the force acting on an object due to gravity. It depends on the mass of the object and the gravitational field strength where it is.
The equation is weight = mass × gravitational field strength, W = mg. Mass is in kilograms, gravitational field strength in newtons per kilogram, and weight in newtons.
Mass and weight are not the same. Mass is a scalar measuring the amount of matter, and it is the same everywhere. Weight is a vector force, and it changes with location — an object on the Moon has the same mass as on Earth but weighs about one sixth as much, because the Moon's gravitational field strength is weaker.
Revision notes
The equation
Weight = mass × gravitational field strength, W = mg.
Mass in kilograms, gravitational field strength in N/kg, weight in newtons. On Earth g is about 9.8 N/kg, though 10 N/kg is often used.
Mass versus weight
Mass is a scalar measuring the amount of matter in an object, and it does not change with location.
Weight is a vector force due to gravity, and it changes with the gravitational field strength. The two are directly proportional at a given location.
Centre of mass
The weight of an object is considered to act at a single point called its centre of mass.
Weight is measured with a calibrated spring balance, sometimes called a newtonmeter.
Key points
- Weight is the force due to gravity.
- Weight = mass × gravitational field strength.
- Weight is measured in newtons.
- Mass is measured in kilograms.
- Mass is the same everywhere; weight changes.
- Weight acts at the centre of mass.
Worked examples
Example 1
Calculate the weight of a 60 kg person on Earth. Take g = 9.8 N/kg. [2 marks]
Model answer
Weight = mass × gravitational field strength = 60 × 9.8 (1 mark) = 588 N (1 mark).
Example 2
Explain the difference between mass and weight. [3 marks]
Model answer
Mass is a scalar quantity measuring the amount of matter in an object, and it is the same everywhere (1 mark). Weight is a vector force caused by gravity, measured in newtons (1 mark). Weight changes with the gravitational field strength, so an object weighs less on the Moon than on Earth although its mass is unchanged (1 mark).
Example 3
An object has a mass of 5 kg. Calculate its weight on the Moon, where g = 1.6 N/kg. [2 marks]
Model answer
Weight = mass × gravitational field strength = 5 × 1.6 (1 mark) = 8 N (1 mark).
Common mistakes
Using kilograms as a unit of weight.
Weight is a force measured in newtons; kilograms measure mass.
Saying mass changes on the Moon.
Mass is the same everywhere; only the weight changes.
Forgetting the gravitational field strength.
The equation has three quantities, not two.
Treating weight as a scalar.
Weight is a force and therefore a vector.
Exam tips
- Always state newtons for weight and kilograms for mass.
- Check whether g is given as 9.8 or 10.
- Explain the mass–weight difference through location.
- Say weight acts at the centre of mass if asked.
Key terms
- Weight
- The force on an object due to gravity, in newtons.
- Mass
- The amount of matter in an object, in kilograms.
- Gravitational field strength
- The force per kilogram, in N/kg.
- Centre of mass
- The single point at which weight is considered to act.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.