Gravity & weight
The idea
Things fall because Earth pulls on them — that much you have seen your whole life. The new idea is splitting two words people mix up daily: mass and weight. Mass is the amount of matter in an object, measured in kilograms, and it is the same everywhere in the universe. Weight is the force of gravity pulling on that mass, measured in newtons, and it depends on where you are standing. The link is simple: weight = mass × gravitational field strength, and near Earth's surface that strength is about 10 N for every kilogram.
The classic misconception is that mass and weight are the same thing because, on Earth, they always change together. Travel breaks the illusion: an astronaut on the Moon has exactly the same mass as at home — same atoms, same difficulty to shove around — but weighs only about one-sixth as much, because the Moon's gravity is weaker. So when a problem says an object 'weighs' some number of newtons, it is telling you about a force, and when it gives kilograms, it is telling you about mass; keep the two in separate mental boxes.
Worked example
An astronaut has a mass of 60 kg. Using 10 N/kg for Earth's gravitational field strength and 1.6 N/kg for the Moon's, find her weight in both places.
- On Earth, weight = mass × field strength = 60 kg × 10 N/kg = 600 N. This is the downward force Earth's gravity exerts on her.
- On the Moon the rule is the same but the field strength is smaller: weight = 60 kg × 1.6 N/kg = 96 N.
- Note what did NOT change: her mass is 60 kg in both places, because moving location does not add or remove matter.
- Sanity-check the ratio: 96 N ÷ 600 N = 0.16, about one-sixth — exactly what you expect, since the Moon's gravity is roughly one-sixth of Earth's.
Answer. She weighs 600 N on Earth and only 96 N on the Moon, while her mass stays 60 kg in both places.
Check your understanding
- Why does a bathroom scale give a misleading reading if you think of it as measuring mass rather than force?
- How would you convince a friend that an astronaut floating in orbit still has all of their mass?
- What would happen to your weight and your mass on a planet with gravity three times stronger than Earth's?
- Why do heavy and light rocks dropped together hit the ground at nearly the same time, even though gravity pulls harder on the heavy one?
Build the foundations first
Gravity & weight builds on these concepts. If any feel shaky, start there.