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Mass vs weight
Beginner
Mass is the amount of matter; weight is the gravitational force on it.
W = mgMass measures how much matter a body contains and how strongly it resists acceleration; it is the same everywhere. Weight is the gravitational force on that mass and varies with place: W = mg. A balance compares masses, while a spring scale measures weight.
A 60 kg person weighs about 589 N on Earth (g = 9.81) but only about 98 N on the Moon (g ≈ 1.63), yet their mass is still 60 kg.
Everyday "weight in kg" really means mass. In physics equations, use newtons for weight.
In more detail
Mass is the amount of matter in an object and stays the same everywhere. Weight is the gravitational force on that mass, W = mg, so it changes with the local value of g.
Real-world uses
- Astronauts have the same mass on the Moon, but a smaller weight.
- Bathroom scales measure weight, which is why they show different readings on different planets.
- Engineers design lifting cranes using weight in newtons, but the same crane needs its mass in kilograms to predict how it accelerates.
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Frequently asked questions
What is the SI unit of mass?
The kilogram. The newton is the unit of weight, which is a force.
Do I weigh less at the top of a mountain?
Very slightly, because g is a little smaller at higher altitude, but mass stays the same.
Do astronauts become weightless or massless?
They become weightless in orbit, but their mass is unchanged. Weight is the force from gravity, which is still present, and it is not zero at orbital height.
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