Gravity, Mass and Weight
Mass and weight are not two words for the same thing. One is how much stuff you are made of and the other is how hard gravity pulls on it, and only one of them changes when you leave Earth.
What gravity is
Gravity is a pull between any two objects that have mass.
Every object pulls on every other one. You are pulling on the Earth right now, with exactly as much force as it is pulling on you. The reason you move towards it and it does not visibly move towards you is that it has rather more mass than you do.
The pull gets stronger when the objects have more mass, and weaker when they are further apart. That is why the Sun holds the planets in place across enormous distances, and why you do not get pulled sideways by the person sitting next to you.
Mass against weight
Two different quantities
Mass is how much matter something contains. Measured in kilograms. It is the same everywhere in the universe.
Weight is the force of gravity pulling on that mass. Measured in newtons. It changes with where you are.
Everyday speech does not keep these apart at all. A bathroom scale says kilograms and we call the reading our weight, and nobody in a shop is going to thank you for correcting them. In science the two are separate, and questions are written specifically to find out whether you know that.
The same bag on the Moon
Take a 6 kg bag to the Moon and back and nothing about the bag changes. There is exactly the same amount of matter in it. But the Moon has far less mass than Earth, so it pulls much more weakly.
An astronaut can carry equipment on the Moon that they could not lift on Earth. Not because the equipment became less real, but because there is less pulling it down.
Working weight out
Weight = mass x gravitational field strength
On Earth the field strength is about 10 N per kg, and on the Moon about 1.6 N per kg.
So a mass in kilograms, multiplied by ten, gives roughly its weight on Earth in newtons.
A dog has a mass of 25 kg. What is its weight on Earth?
- Weight = mass x field strength.
- The mass is 25 kg and Earth's field strength is about 10 N per kg.
- 25 x 10 = 250.
- Weight is a force, so the unit is newtons.
250 N
The same dog is taken to the Moon. What are its mass and weight there?
- Mass does not change with location, so it is still 25 kg.
- The Moon's field strength is about 1.6 N per kg.
- 25 x 1.6 = 40.
- So the same dog, the same matter, a much smaller pull.
Mass 25 kg, weight 40 N
Common mistakes
1. Saying mass changes on the Moon
The commonest answer, and it comes straight from everyday language rather than from muddled thinking. Nothing is added to or taken from the object by moving it, so the mass cannot change. Only the pull does.
2. Giving weight in kilograms
Weight is a force, and forces are measured in newtons. Writing "weight = 60 kg" loses the mark even when the arithmetic is right, which is a frustrating way to lose one. Kilograms for mass, newtons for weight, every time.
3. Thinking there is no gravity in space
Astronauts float, so it looks obvious. But the Space Station is only a few hundred kilometres up and gravity there is nearly as strong as on the ground. They float because they are falling around the Earth continuously, along with everything else in the station.
Practice questions
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What to learn next
Once forces make sense, machines are the interesting question: a lever lets you move a load with less force than its weight, and it is worth knowing why that is not cheating.
For parents
Ask your child what a bathroom scale actually measures. It is a genuinely good question with an awkward answer: it measures the force you press down with, then converts it to kilograms assuming Earth's gravity. Take it to the Moon and it would read wrong, which makes the mass and weight distinction concrete rather than a definition to learn.
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