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Density.

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 m^{3}.

If the same
10 kg of substance is
squashed into a

volume of 0·5 m^{3},
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 m^{3}

In the second case

d =
10 ÷ 0·5

= 20 kg
per m^{3}

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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