Why mass and weight are different quantities
Mass measures an object's inertia, while weight is the gravitational force acting on it. The same object can have different weights in different gravitational fields without changing its mass. In physics, mass is measured in kilograms and weight in newtons, even though everyday language often mixes them.
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One object, two measurements
An object's mass describes its resistance to acceleration. Its gravitational weight depends on both its mass and the local gravitational field strength.
The relationship is weight = mass × gravitational acceleration, commonly written as W = mg.
Near Earth's surface, a mass of one kilogram has a gravitational weight of roughly 9.8 newtons. In a weaker gravitational field, the same mass has a smaller weight.
- Kilograms are units of mass.
- Newtons are units of force.
- Changing the gravitational field changes weight without necessarily changing mass.
A bathroom scale measures a supporting force and converts the result into a mass reading using assumptions about gravity. Accelerating motion can alter that supporting force, so the displayed value need not always represent a real change in the person's mass.