# WEIGHT vs MASS

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The story so far...
All masses attract all other masses to a certain extent.
The earth, being a very large mass, exerts a very large force ot attraction which is called the force of gravity.
This pulls all objects towards the earth at an acceleration of roughly 10 metres per second per second.
The force of gravity acting on a mass of 100 grams is called one newton.
And now ...
WEIGHT vs. MASS.
If anybody ever asks you what a newton is ...
Yes, you could give them an apple,
or a couple of golf balls for that matter,
or a small flashlight battery,
because all of these have a mass of about 100 grams.
And, when the earth pulls them down with and acceleration of 10 metres per second per second,
we say that the force of gravity acting on them is one newton,
or that the apple, two golf balls and the flashlight battery are all pulled down to earth by a force of one newton.
How many newtons are pulling you down to earth?
Well, the simplest way to measure a force is to use a newton spring scale.
If we hang the apple on it,
we see that the earth is pulling the apple down with a force of one newton,
and if we hang you on the spring scale,
we see that the earth is pulling you down with a force of...
let's see ...
just about 600 newtons.
So now we know how much you weigh.
What you weigh merely means what force of gravity is pulling you down to earth.
That's all weight is:
The force of gravity.
Remember how keen physicists are on Latin words?
Well, 'gravity' comes from 'gravitas',
which is simply Latin for weight.
But you had your own weight in kilograms, didn't you?
That's true in every day life,
but not in physics,
which has to be very careful about these things.
In physics, kilograms measure your mass
on a balance,
but your weight is always measured in newtons on a spring scale.
You'll have noticed by now that you can usally work out the number of newtons
by multipling the kilograms by ten.
100 grams or one tenth of a kilogram is about the same as one newton,
and 60 kilograms are about the same as 600 newtons.
But if one kilogram is the equivalent of ten newtons,
why bother to make a distinction between a mass of 60 kilograms and a weight of 600 newtons,
if they both really come to the same thing?
To find the answer to that, let's go to the moon.
It doesn't feel as if you weigh so much here, does it?
Let's see how many newtons are pulling you down on the moon.
Only a hundred.
Why is that?
Because the moon is much smaller than the earth,
and therefore has much less mass,
so its force of gravity is also much smaller,
only one sixth that of the earth in fact.
This is why your weight on the moon is only one sixth of you weight on earth.
What about your mass?
On earth, it is 60 kilograms.
How much will it be on the moon?
60 kilograms.
Exactly the same, of course.
You don't get any smaller by going to the moon.
You don't change!
Only the force of gravity, and therefore your weight, changes.
That's the big difference between mass and weight.
Wherever you go in the universe,
your mass stays the same.
Only your weight changes.
So if you're not as slim as you'd like to be,
don't eat so much.
You should join the Mass Watchers Club
because you've got a mass problem.
But if all you want to do is lose weight,
go to the moon.