PHYSICS · MASS & WEIGHT

Your mass never changes. Your weight does — the moment you leave Earth.

Pack a 10 kg suitcase on Earth and fly it to the Moon: it's still 10 kg of stuff. But a scale under it would read six times lighter. That's the whole difference in one trip.

Two different questions

"How much matter is in this object?" and "how hard is gravity pulling on it right now?" sound similar. They aren't.

The unchanging one

Mass

kilograms (kg) · scalar

A measure of how much matter — atoms, stuff — is packed into an object. It doesn't care where the object is. A brick has the same mass in your kitchen, in orbit, or drifting in deep space with no gravity at all.

Measured with: a balance, comparing an object against known masses.

The one that depends on where you are

Weight

newtons (N) · vector, points toward the planet

The force gravity exerts on that mass. Since gravity's strength changes from planet to planet — and even slightly place to place on Earth — the same object weighs differently depending on where it's sitting.

Measured with: a spring scale, which reads the force pulling down on it.

Weight=mass×gravity → W=mg

g is ~9.8 m/s² on Earth — but it's a variable, not a constant, once you leave the planet.

Try it: same mass, different planet

Set an object's mass once. Then move it around the solar system and watch only the weight react.

Mass

10 kg

Weight

98.1 N

Object mass — 10 kg

Location

The mass block stays the same size the whole time — that's the point. Only the spring's stretch, which stands in for the weight force, changes as gravity changes.

Side by side

Every distinction traces back to one fact: mass is a property of the object; weight is a property of the object and the gravity it's sitting in.

	Mass	Weight
What it measures	Amount of matter	Force of gravity on that matter
SI unit	kilogram (kg)	newton (N)
Type of quantity	Scalar (just a size)	Vector (a size, pointing down)
Changes with location?	No — same everywhere	Yes — depends on local gravity
Value in zero gravity	Unchanged	Zero
Typical instrument	Balance / beam scale	Spring scale