O Level / IGCSE Physics · Motion, forces and energy. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
Mass is a measure of the quantity of matter in an object. It is measured in kilograms and it is the same everywhere. Mass also gives an object inertia: the larger the mass, the harder it is to start it moving, stop it or change its direction.
Weight is the force of gravity on an object. It is measured in newtons and it changes from place to place, because it depends on the gravitational field strength there. A gravitational field is a region where a mass feels a force due to gravitational attraction.
An electronic balance gives mass. A force meter (spring balance) measures weight. A beam balance compares two weights, and so two masses.
Giving a weight in kilograms, or changing the mass when the object goes to another planet. Mass stays the same; only the weight changes.
Answer: 125 N (the mass is still 5.0 kg).
Try each one, then open the answer.
Answer: D. Mass is the amount of matter and is the same everywhere; it is measured in kilograms. Weight is the gravitational force and changes with g.
Answer: C. weight = mass × gravitational field strength = 2.5 × 10 = 25 N.
Answer: B. Inertia is the resistance of a mass to a change in its motion. The larger the mass, the larger the inertia.