Weight or Mass?

Q: Aren't "weight" and "mass" the same?

A: Not really.

 weight normally shows 100  

An object has mass (say 100 kg).

This makes it heavy enough to show a weight of "100 kg".

But the scales are only showing a guess of the mass above them!

Gravity causes Weight


An object's weight is how hard gravity is pulling on it.

We think the weight is the same everywhere ... because we all live on the surface of the planet Earth!

freefall weight is zero

But in orbit it would not push on the scales at all.

The scales would show 0 ...
... but the mass is still 100 kg !

An object's mass doesn't change (unless you remove some!), but its weight can change.

freefall weight is zero

On the Moon the scales would wrongly show 16.6
for a mass of 100 kg

Because the pull of gravity on the Moon
is much less than on Earth

So Why Do People Say Weight instead of Mass?

People often use "weight" to mean "mass", and vice versa, because Gravity is almost the same everywhere on Earth and we don't notice a difference.

But remember .. they do not mean the same thing,
and they can have different measurements.

Weight is a Force

So ... if weight and mass are different, why are they both in kilograms?

Well, weight should not really be in kilograms!

I have used "kilogram" so far because that is what you see on a pair of scales, but it is technically wrong to talk about weight in kilograms ...

... weight is a force ...

... which is measured in Newtons


The correct unit for force is the Newton (=1 kg·m/s2) which is abbreviated N.

down On the Earth's surface gravity makes a
1 kilogram mass exert about 9.8 Newtons of force

So a 100 kg mass really weighs about 980 Newtons on Earth.


scales in newtons

Why Do Scales Show Kilograms?

Scales show Kilograms because that is what people understand best ...

... but it is really just an estimate of the mass above them.

Scales should really show Newtons, but that might confuse people!


Question: how many Newtons should the scales show when you stand on them (hint: multiply kg by 9.8)?


Apparent Weight

But scales can be fooled ... because they measure a "downwards force" and don't know if it is gravity or some other force!

Just jump up and down (gently!) on your scales at home to see your apparent weight change, while your mass stays the same.

So your mass is the same, and your weight is the same (because the force of gravity hasn't changed), but your "apparent" weight changes. Read more at Apparent Weight