Weight or Mass?

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

A: Not really.

An object has mass (say 100 kg).

This makes it heavy enough to weigh 100 kg

Gravity causes Weight

gravity

An objects 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 kg ... but the mass is still 100 kg !

  freefall weight is zero

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

 

So Why Do People Say Weight instead of Mass?

People often use "weight" to mean "mass", and vice versa.

Because gravity is pretty much the same everywhere on Earth, we don't notice a difference.

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

Here are some conditions where the Weight might change:

  • in space (can be weightless!)
  • on the moon (a 100 kg mass would weigh 16.6 kg)
  • you can even get very slight differences in weight in different locations on earth!

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 would see on a pair of scales, but it is technically wrong to talk about weight in kilograms ...

... but there is a better measurement: Newtons

Newtons

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

down Gravity makes a 1 kilogram mass exert about 9.8 Newtons of force

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

scales in newtons

Why Scales Show Kilograms or Pounds

But scales show Kilograms or Pounds 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 downwards
  • Apparent Weight is a measure of downwards force
  • Force is measured in Newtons, not kilograms or pounds
  • When scales show "kg" or "lb" it is just an estimate of the mass above them