What Is Weight?

Weight, a Force

Weight is the force of attraction exerted by a planet (or celestial body) on an object, due to gravity. Unlike mass, weight is a force, not a quantity of matter.

Unit and Measuring Instrument

Weight is measured in newtons (N), the unit of force in physics. It is measured with a dynamometer, an instrument different from the scale used to measure mass.

Quantity	Symbol	Unit	Instrument
Mass	m	kilogram (kg)	scale
Weight	P	newton (N)	dynamometer

Weight Depends on Location

Unlike mass, the weight of an object changes depending on where it is, because gravity is not the same everywhere. On the Moon, where gravity is weaker, an object weighs about 6 times less than on Earth, even though its mass does not change.

Concrete Example

A 10 kg bag on Earth has a weight of about 100 N. The same bag carried to the Moon would still have a mass of 10 kg, but a weight of only about 16 N: it would therefore be much easier to lift!

Common Pitfall

Never say "weight is measured in kg": this is a common mistake. Weight is always measured in newtons (N).