# Definition:Weight (Physics)

## Definition

The weight of a body is the magnitude of the force exerted on it by the influence of a gravitational field.

The context is that the gravitational field in question is usually that of the Earth.

### Weigh

To weigh a body is to determine its weight, and thence its mass.

Similarly we can say that:

body $B$ weighs $x$

to mean:

the weight of the body $B$ is $x$

and both mean the same thing.

### Dimension of Weight

The dimension of weight is $\mathsf M \mathsf L \mathsf T^{-2}$: mass times acceleration, that is, a force.

### Units of Weight

The SI unit of weight is $\mathrm N$ (newton).

The CGS unit of weight is $\mathrm {dyn}$ (dyne).

## Warning

There is a certain amount of confusion in the common mind between weight and mass.

The latter is usually determined by measuring its weight.

But while the mass of a body is (under normal circumstances) constant, its weight varies according to its position relative to the gravitational field it is in, and so is not a constant property of that body.

However, under usual terrestrial conditions the gravitational field is more or less constant (any differences being detectable only by instruments).

This means that the weight and mass of a body are commonly considered "the same".

Thus a weighing machine, while indicating the mass of a body, does so by measuring its weight.