# Weight or Mass?

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

**A**: Not really.

An object This makes it heavy enough to show a |

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!

**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**.

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**

## Newtons

The correct unit for force is the **Newton** (=1 kg·m/s^{2}) which is abbreviated **N**.

On the Earth's surface gravity makes a1 kilogram mass exert about 9.8 Newtons of force |

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

### 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)?*

- So the scales show an
**estimate of your mass**based on the force your body exerts on it. - And to find out how much force your body is exerting on the scales, multiply by 9.8 (to convert kg into Newtons).

## 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

## Conclusion

**Mass**is a measure of how much**matter**something contains**Weight**is a measure of how strongly gravity pulls**Apparent Weight**is a measure of downwards force**Force**is measured in**Newtons**, not kilograms- When scales show "kg" it is just an
**estimate**of the mass above them