Mass, weight and gravity
Physics · Key Stage 3 · The School
Two different quantities with one everyday name
Mass is how much matter something contains, measured in kilograms, and it does not change wherever you take it. Weight is the force gravity pulls on that mass with, measured in newtons, and it does change. In ordinary speech both are called weight, which is why the physics feels like hair-splitting until you leave the Earth.
Weight = mass × gravitational field strength
On Earth g is about 10 N/kg, so a 60 kg person weighs about 600 N. On the Moon g is about 1.6 N/kg, so the same person weighs about 96 N — and still has a mass of 60 kg, because none of their matter was left behind. The formula is the whole relationship, and it is worth writing out rather than remembering a conclusion.
What a bathroom scale actually measures
It measures force — how hard you push down on it — and then divides by g and prints kilograms, because that is what people want to see. Take the same scale to the Moon and it would read about 10 kg for a 60 kg person. Nothing about the person changed; the assumption built into the scale did.
Astronauts are not weightless
On the space station gravity is still roughly 90% of its strength at the ground. Astronauts float because they and the station are both falling around the Earth at the same rate — continuous free fall, not an absence of gravity. “Weightless” is a description of the sensation, not of the physics, and it is the single most misleading word in the topic.