The difference between weight and mass lies in their definitions and how they behave under different conditions:

• Mass is the amount of matter contained in an object. It is a measure of how much "stuff" or atoms make up the object and is directly related to the object's inertia, which is its resistance to changes in motion. Mass is a scalar quantity and remains constant regardless of location. The SI unit of mass is the kilogram (kg)

• Weight is the force exerted on an object due to gravity. It depends on both the mass of the object and the gravitational acceleration acting on it. Weight is a vector quantity (a force) measured in newtons (N) in the SI system. Because gravitational acceleration varies with location (e.g., Earth, Moon, other planets), weight changes depending on where the object is, while mass remains the same

The relationship between weight and mass is given by the formula:

Weight=Mass×Gravitational acceleration(W=m×g)\text{Weight}=\text{Mass}\times \text{Gravitational acceleration}\quad (W=m\times g)Weight=Mass×Gravitational acceleration(W=m×g)

For example, an object with a mass of 1 kilogram weighs about 9.8 newtons on Earth's surface but would weigh less on the Moon due to weaker gravity, even though its mass is unchanged

. In everyday language, people often use "weight" to mean mass (e.g., saying "70 kilograms" to describe body weight), but scientifically, weight and mass are distinct concepts. Summary:

Aspect| Mass| Weight
---|---|---
Definition| Amount of matter in an object| Force due to gravity on the object
Quantity type| Scalar| Vector (force)
Unit (SI)| Kilogram (kg)| Newton (N)
Depends on| Amount of matter only| Mass and gravitational acceleration
Changes with location?| No| Yes
Measured by| Balance (comparison of matter)| Scale (measures force)

This distinction explains why your mass stays the same everywhere in the universe, but your weight varies depending on the gravitational pull of the celestial body you are on