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Forces and Motion


What is Density?

Density = mass ÷ volume.

This equation is important!

It is important to know the difference between density and weight.

If we have 10 kg of a substance,
its mass is 10 kg and its weight is 100 Newtons.
The mass is the amount of stuff you have (the number of atoms).
The weight is the force of gravity pulling on that mass.
Imagine that the 10 kg of substance takes up a volume of 1 m3.

If the same 10 kg of substance is squashed into a
volume of 0·5 m3, then we still have the same mass
and it still has the same weight but it now only
takes up half the space. The density has been doubled.

How can we Calculate Density?

If we do the calculation using d = m ÷ v,

In the first case above
d = 10 ÷ 1
             = 10 kg per m3

In the second case
d = 10 ÷ 0·5
          = 20 kg per m3

The same mass in half the volume gives twice the density.

Sometimes the Greek letter ρ is used for density, instead of d.
ρ looks like a p but is in fact a Greek r (called rho).

ρ = m ÷ v

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