Forces and Motion

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, it still has the same weight
but it now only takes up half the space.
The density has been doubled.


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

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

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

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

r = m ÷ v

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