Ask most people their weight and they will give you a number in kilograms. To a physicist, though, that answer is technically a measure of mass, not weight. So what is the difference between mass and weight? In short: mass is how much stuff something is made of, and weight is how hard gravity pulls on that stuff.
| Mass | Weight | |
|---|---|---|
| What it is | The amount of matter in an object | The force of gravity pulling on that matter |
| Does it change with location? | No, it is the same everywhere | Yes, it depends on the local gravity |
| Unit | Kilograms (kg) | Newtons (N) |
| Formula | Mass = weight ÷ gravity | Weight = mass × gravity (W = mg) |
| Measured with | A balance, comparing against known masses | A spring scale, measuring force |
What mass is
Mass is a measure of how much matter, or stuff, is in an object. It also tells you how hard something is to get moving or to stop: push a shopping trolley full of groceries and an empty one equally hard, and the empty one speeds up much more, because it has less mass. Your mass stays the same wherever you go. On the Moon, on Mars or floating in space, you are made of exactly the same amount of you. The international unit of mass is the kilogram.
What weight is
Weight is a force: the pull of gravity on your mass. Because it is a force, physicists measure it in newtons, named after Isaac Newton. On Earth, gravity pulls with about 9.8 newtons for every kilogram, so a person with a mass of 70 kg has a weight of about 690 newtons. Unlike mass, weight changes depending on where you are, because gravity is stronger or weaker in different places. It even varies slightly across Earth: you weigh a tiny bit less at the equator or on top of a mountain than at sea level near the poles.
So what is my bathroom scale measuring?
Strictly speaking, a bathroom scale measures weight: it measures how hard you press down on it. But it then divides by Earth’s gravity and shows you the answer in kilograms, which is your mass. That works fine on Earth, which is why we happily use “weight” and “kilograms” together in everyday life. Take the same scale to the Moon, though, and it would show a number about one-sixth of what it shows at home, even though your mass has not changed at all.
Your weight on other worlds
Because gravity is different on other planets and moons, your weight would change dramatically if you travelled through the solar system, while your mass stayed exactly the same. Here is roughly what a scale would show for someone who reads 70 kg on Earth:
| Where you are | Gravity compared with Earth | What the scale would show |
|---|---|---|
| Earth | 100% | 70 kg |
| The Moon | about 17% | about 12 kg |
| Mars | about 38% | about 27 kg |
| Jupiter (if you could stand on it) | about 250% | about 177 kg |
That is why astronauts on the Moon could bounce around so easily in heavy spacesuits: their mass was the same as on Earth, but their weight was only about a sixth.
Why astronauts float
This is where most people go wrong. Astronauts on the International Space Station float around, so it is easy to assume there is no gravity in space. But the station orbits only about 400 km above the ground, and at that height, Earth’s gravity is still nearly 90% as strong as it is where you are reading this. (Gravity weakens with the square of your distance from Earth’s centre, and 400 km is only a small step out from the roughly 6,400 km you are already sitting at.) So why do they float? Because the station and everyone in it are constantly falling towards Earth, while also moving sideways so fast, about 28,000 km/h, that they keep missing it. Everything falls together, so nothing presses against anything else, and astronauts feel weightless. It is the same feeling as the drop at the top of a rollercoaster, just continuous. Their mass has not changed at all, which is why it still takes effort to push a heavy piece of equipment across the cabin, or to stop it once it is moving.
The kilogram itself changed in 2019
For 130 years, the kilogram was defined by a single metal cylinder, known as Le Grand K, kept under glass jars near Paris; every scale on Earth was ultimately calibrated against it. But tiny changes in its surface meant even it could not be trusted forever. On 20 May 2019, the kilogram was redefined using a fixed value of a fundamental constant of nature called the Planck constant, which never changes, as Science News reports. Your bathroom scale did not notice, but the definition of mass itself is now tied to the laws of physics rather than a lump of metal.
The short version
Mass is how much stuff you are made of, and it is the same everywhere in the universe. Weight is how hard gravity pulls on that stuff, and it changes depending on where you are. In everyday life, on Earth, the difference rarely matters, which is why “weight” and “kilograms” get used together all the time. But step onto the Moon, or into orbit, and the difference becomes impossible to ignore. Mass and weight are a lot like speed and velocity, another pair of words we use interchangeably that mean different things to a physicist, as I explain in speed vs velocity: what is the difference? And for more on how gravity shapes the way things move through space, see my post on the barycentre.
Sources
- Science News: It’s official, we’re redefining the kilogram
- NASA Space Place: How GPS and orbiting spacecraft are affected by gravity