Distinguish between mass and weight. A body has a mass of 12 kg. Find its weight on the earth and on the moon. (g on the earth = 9.8 m s⁻², g on the moon = 1.63 m s⁻²)
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define mass. Mass is the quantity of matter in a body. Its unit is the kg, and it is the same everywhere.
2 · Define weight. Weight is the force of gravity on a body, W = mg. Its unit is the newton, and it changes with g.
3 · Two more differences.
Mass is measured with a pan balance; weight with a spring balance.
Mass has size only; weight has size and a downward direction.
4 · Weight on the earth. W = mg = 12 × 9.8 = 117.6 N.
5 · Weight on the moon. W = 12 × 1.63 ≈ 19.6 N, about one sixth. The mass stays 12 kg.
6 · Close. So mass is fixed, but weight depends on where the body is.
Scoring tip: Give mass in kg and weight in N. Say clearly that the mass does not change on the moon.
