Edexcel IGCSE Physics · Spec 1.5
Mass, Weight & Gravity
The difference between mass and weight, and gravitational field strength.
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Mass, Weight & Gravity
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Explained
Mass and weight, and the number that connects them
In everyday speech these two words mean the same thing. In physics they are different quantities with different units, and a good many marks across the whole course depend on keeping them apart.
Mass
Mass is the amount of matter in an object, measured in kilograms. It is a scalar, so it has size but no direction.
Crucially, it does not change when the object moves. A 60 kg astronaut has a mass of 60 kg on Earth, on the Moon and floating in deep space, because the amount of matter has not changed. Mass is measured with a balance, which compares one mass against another.
Weight
Weight is the force acting on a mass because of a gravitational field, measured in newtons. It is a vector, and its direction is towards the centre of the planet.
Weight does change with location, because the gravitational field strength changes. The same astronaut weighs about 600 N on Earth and about 100 N on the Moon. Weight is measured with a newtonmeter, which is a spring balance.
The equation
Weight equals mass times gravitational field strength, W equals m g.
Gravitational field strength is measured in newtons per kilogram, and the unit says what the quantity means: how many newtons of force act on each kilogram of mass. On Earth it is about 10 N/kg, so each kilogram is pulled with about 10 N of force.
Mass must be in kilograms. A mass given in grams has to be converted first, and that conversion is the most common reason a weight comes out a thousand times too large.
Why g differs from place to place
Gravitational field strength depends on the mass of the planet and on the distance from its centre. The Moon has far less mass than the Earth, so its field is weaker, so anything on its surface weighs less.
Note that g has a second meaning that is really the same one. It is also the acceleration of a freely falling object, which on Earth is about 10 m/s squared. Those two are numerically equal because weight equals mass times g and force equals mass times acceleration, so the acceleration produced by gravity alone is g.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme asks why the gravitational field strength on the Moon differs from that on Earth. It credits three ideas: that the Moon and Earth have different masses, that gravitational field strength is proportional to mass, and that the field strength on the Moon is less.
The notes are unusually forgiving here. They allow density in place of mass, they allow the converse statement throughout, and they condone the loose phrasings Earth is heavier, Moon is lighter, and less gravity on the Moon.
That generosity has a limit, though, and it is worth reading carefully. The marks are for comparing the two bodies and linking the comparison to field strength. An answer that only says the Moon is smaller, without connecting size or mass to the strength of the field, has not made the link the question is testing.
The examiner report on that paper adds that most candidates gained one mark for appreciating the difference in masses, but only the most able gained a further mark for saying how the field strengths differ. The comparison is the easy mark; the consequence is the harder one.
Answering the difference question
Give both quantities, both units, and the behaviour that separates them. Mass is the amount of matter, measured in kilograms, and it is the same everywhere. Weight is the force of gravity on that mass, measured in newtons, and it changes with gravitational field strength.
Three contrasts, three marks, and the last one is the one that shows you understand rather than remember.
Spec 1.5
What you need to know
- Tell the difference between mass and weight
- Use weight equals mass times gravitational field strength
- Explain what g means
Active recall
Quick check
Answer each question before opening the answer.
What is the difference between mass and weight?
Mass is the amount of matter (kg) and is constant; weight is the force of gravity on that mass (N) and depends on gravitational field strength.
What is the equation for weight?
Weight = mass × gravitational field strength (W = m × g).
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