Mass and Weight

Two words often mixed up

In everyday life, people say "I weigh 40 kilograms." But in science, that’s not quite right. Kilograms measure mass, not weight. They are related, but they are different things.

Mass

Mass is how much matter, or "stuff," is in something. Your mass is the same whether you’re in Canada, on the Moon or floating in space.

Mass also measures inertia: how hard it is to start or stop something moving. A truck has more mass than a bicycle, so it’s harder to push and harder to stop.

Units of mass:

• 1 kilogram (kg) = 1000 grams (g)
• 1 tonne (t) = 1000 kg

Weight

Weight is the force of gravity pulling on an object’s mass. Since it’s a force, it’s measured in newtons (N).

weight = mass × gravitational field strength

W = m × g

On Earth, gravitational field strength (g) is about 9.8 N/kg. That means each kilogram of mass is pulled down with about 9.8 newtons of force.

Example: A student with a mass of 40 kg has a weight of 40 × 9.8 = 392 N on Earth.

Weight on other worlds

Different planets and moons have different gravity, so your weight changes, but your mass doesn’t.

On the Moon, you could jump about six times higher than on Earth! Astronauts in the Apollo missions bounced across the Moon’s surface. In 1971, Apollo 15 astronaut David Scott dropped a hammer and a feather on the Moon. With no air, they hit the ground at the same time.

Measuring mass and weight

Balance (measures mass)

A balance compares an object with known masses. Because gravity pulls equally on both sides, a balance gives the same reading anywhere there’s gravity, even on the Moon.

Spring scale (measures weight)

A spring scale measures how much gravity pulls an object down by how far a spring stretches. On the Moon, the spring would stretch less, giving a smaller reading.

Bathroom scales actually measure weight (force) but display it as mass, assuming you’re on Earth.

Weightlessness

Astronauts on the International Space Station seem weightless. But gravity there is still about 90% as strong as on Earth’s surface! They float because they and the station are falling around Earth together in orbit. This is called free fall. Their mass hasn’t changed at all.

Why do heavier objects not fall faster?

A heavier object has more weight (a bigger pull), but it also has more mass (more inertia to overcome). The two effects cancel, so all objects fall with the same acceleration, 9.8 m/s², when there’s no air resistance. Galileo is said to have shown this in the late 1500s.

On Earth, a feather falls slowly because of air resistance, not because it’s light.

Common mistakes

• Saying weight in kilograms. Weight is a force, measured in newtons.
• Thinking mass changes on the Moon.
• Thinking there’s no gravity in space. There’s gravity almost everywhere; astronauts are in free fall.

Try it yourself

1. Calculate your weight in newtons on Earth, the Moon and Mars.
2. Hang an object from a rubber band and measure the stretch. Add a second identical object. How does the stretch change?
3. Drop a book and a sheet of paper at the same time. Then place the paper on top of the book and drop them. Explain the difference.

Quick recap

• Mass is the amount of matter (kg); weight is the pull of gravity (N).
• W = m × g; on Earth, g ≈ 9.8 N/kg.
• Mass stays the same everywhere; weight changes with gravity.
• Balances measure mass; spring scales measure weight.
• Astronauts float because they’re in free fall, not because gravity is gone.