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Cambridge IGCSE Physics · 0625

Chapter 1: Motion, forces and energy — Part 3

Topic 1.3 · Mass and weight

Mass

Mass is a measure of the quantity of matter in an object at rest relative to the observer.

Mass is a scalar. Its SI unit is the kilogram (kg). Mass does not change if the object is taken to the Moon.

Weight and gravitational field strength

Weight is a gravitational force on an object that has mass. Weight is a vector (it acts towards the centre of the planet). Unit: newton (N).

Gravitational field strength g is force per unit mass:

g = W / m   so   W = mg.

This g is equivalent to the acceleration of free fall (about 9.8 N/kg or 9.8 m/s2 near Earth).

A student has mass 65 kg. Calculate her weight on Earth. Take g = 9.8 N/kg.

Worked example: weight on Earth is 65 times 9.8 equals 637 newtons
W = mg = 65 × 9.8 = 637 N. Weight is a force, so the unit is N, not kg.

Exam Traps

  • Do not write weight in kilograms — mass is in kg; weight is a force in N. W = mg, not W = m.

Weight as the effect of a gravitational field

A gravitational field is a region where a mass experiences a force. Weight is the effect of that field on a mass.

On the Moon, mass is the same but g is smaller, so weight is smaller.

The same student (mass 65 kg) stands on the Moon where g = 1.6 N/kg. Calculate her weight on the Moon.

Worked example: weight on the Moon is 65 times 1.6 equals 104 newtons; mass is unchanged
Mass stays 65 kg. W = 65 × 1.6 = 104 N.

Weights (and masses) may be compared using a balance.

The same object on Earth and on the Moon: mass unchanged; weight shown as a downward force arrow, smaller on the Moon because g is smaller.
Diagram 1: The same object on Earth and on the Moon: mass unchanged; weight shown as a downward force arrow, smaller on the Moon because g is smaller.

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