IGCSE Physics: Mass and Weight Practice Questions
Mass is the quantity of matter in an object, measured in kilograms, and it does not change with location. Weight is the gravitational force acting on that mass, measured in newtons, and it is calculated using W = mg.
Sub-topic 1.3 of Cambridge IGCSE Physics 0625 is short, but the mass and weight distinction is tested throughout the whole paper and in Biology and Chemistry contexts too. The questions below cover the calculation, the distinction and the idea of inertia.
What you need to know for Mass and Weight
- MassA measure of the quantity of matter in an object at rest, measured in kilograms. It is the same everywhere in the universe and is measured with a balance.
- WeightThe gravitational force acting on an object that has mass, measured in newtons. It varies with location because gravitational field strength varies.
- The equationW = mg, where W is weight in newtons, m is mass in kilograms and g is the gravitational field strength in N/kg. On Earth g is approximately 9.8 N/kg.
- Gravitational field strengthThe force per unit mass acting on an object in a gravitational field, measured in N/kg. It is numerically equal to the acceleration of free fall in m/s2.
- Measuring each quantityMass is measured with a balance by comparison with known masses. Weight is measured with a calibrated spring balance or newtonmeter, which measures force directly.
- InertiaThe tendency of an object to resist a change in its state of rest or motion. The greater the mass, the greater the inertia.
IGCSE Physics Mass and Weight questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
An astronaut has a mass of 72 kg. Calculate her weight on Earth where g is 9.8 N/kg, and on the Moon where g is 1.6 N/kg. State what happens to her mass on the Moon.
Show the worked answer
- Use W = mg.
- On Earth: W = 72 x 9.8 = 705.6, which is 706 N to 3 significant figures.
- On the Moon: W = 72 x 1.6 = 115.2, which is 115 N to 3 significant figures.
- Her mass stays at 72 kg. Mass is the quantity of matter in her body and does not depend on where she is. Only the gravitational field strength has changed.
Explain the difference between mass and weight, including the units and the instrument used to measure each.
Show the worked answer
- Mass is a measure of the quantity of matter in an object. It is measured in kilograms using a balance, and it is the same at every location.
- Weight is the force of gravity acting on that mass. It is measured in newtons using a calibrated spring balance or newtonmeter.
- Weight depends on the gravitational field strength at that location, so the same object has a different weight on the Moon than on Earth.
- The two are linked by W = mg, so weight is proportional to mass at any given location.
A student hangs different masses from a spring balance and plots weight against mass. Describe the shape of the graph and explain how she could use it to find the gravitational field strength.
Show the worked answer
- Since W = mg and g is constant at a given location, weight is directly proportional to mass.
- The graph is therefore a straight line passing through the origin.
- Rearranging W = mg gives g = W divided by m, which is the gradient of the line.
- She should draw the best fit straight line, choose two widely separated points on it, and calculate the gradient as the change in weight divided by the change in mass. The answer is g in N/kg.
Explain why a fully loaded lorry takes longer to stop than an empty one, even when both have the same braking force.
Show the worked answer
- The loaded lorry has a much greater mass than the empty one.
- Rearranging F = ma gives a = F divided by m, so for the same braking force a larger mass produces a smaller deceleration.
- A smaller deceleration means the lorry loses speed more slowly.
- It therefore takes longer to stop and travels a greater braking distance. This resistance to a change in motion is called inertia, and it increases with mass.
Common mistakes in this topic
- Using kilograms as a unit of weight.
- Saying an astronaut is weightless in orbit because there is no gravity. Gravity is still acting. The astronaut is in continuous free fall.
- Forgetting to convert grams to kilograms before using W = mg.
- Treating g as 10 N/kg when the question specifies 9.8 N/kg.
- Confusing the balance with the spring balance when asked to name the instrument.
Exam tips
- Write W = mg before every calculation, then substitute. The equation alone often carries a mark.
- Check the value of g given in the question and use that value, not a remembered one.
- Remember that gravitational field strength in N/kg is numerically the same as the acceleration of free fall in m/s2.
- State clearly that mass is constant whenever a question moves an object to another planet or the Moon.
- Convert all masses to kilograms first. A 500 g mass is 0.5 kg.
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Mass and Weight FAQs
What is the difference between mass and weight?
Mass is the quantity of matter in an object, measured in kilograms with a balance, and it is the same everywhere. Weight is the gravitational force acting on that mass, measured in newtons with a spring balance, and it changes with location because gravitational field strength varies.
How do I calculate weight?
Multiply the mass in kilograms by the gravitational field strength in newtons per kilogram, using W = mg. On Earth g is approximately 9.8 N/kg, so a 10 kg mass weighs about 98 N. Always convert grams to kilograms before substituting.
What is gravitational field strength?
Gravitational field strength is the force per unit mass acting on an object in a gravitational field, measured in newtons per kilogram. It is numerically equal to the acceleration of free fall at the same location, which is why both are given the symbol g.
Does mass change on the Moon?
No. Mass is the quantity of matter in an object and is unchanged wherever the object is. What changes is weight, because the Moon's gravitational field strength is about 1.6 N/kg compared with 9.8 N/kg on Earth, so the same object weighs roughly one sixth as much.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.