# The idea of density.

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The idea of density

Taking up space Which is heavier, a tonne of lead or a tonne of feathers? Everyone knows that lead is heavier than feathers, but that is not the right answer. Tonne of anything weighs the same. 1t=1000kg One tonne of feathers occupies much more space than one tonne of lead.

Comparing materials We say that lead has a greater density than feathers. The density of material tells us about the amount of a substance in one cubic centimetre of the materials. For each material we find the mass of 1cm3.

Explaining density Air has very low density ,ρ=0.0013g/cm3
Particles are spread out, far apart from each other. Lead is a solid. Particles are packed closely together. The particle model tells us why lead has a greater density than air.

Question When water boils, it becomes steam. Its particles spread out and occupy a greater volume. Has its density increased or decreased?

Measuring density Density=mass/volume Mass in grams
Volume in centimeters cubed Mass is measured using a balance. Measuring volume (rectangular solid block): Volume=length x width x height Or volume of liquid with measuring cylinder, read the volume from the scale. Irregular solid- put in the water in the measuring cylinder. Calculate the increase in volume.

Density calculations P:124 Density=mass/volume Mass=density x volume
(Units 1 l= 1dm3 1m3=1000 l)

Q 1. ρ=78kg/m3 ρ=….g/cm3 (7.8g/cm3)
2.V=4,2cm3, ρ=2500kg/m3, m=? (glass spheres-marbles) (m=10,5g) 3. m=3300kg, ρ=2200kg/m3, V=? (1.5m3)-concrete block 4. one object m1=5kg,V1=0.001m3;other object m2=150g, V230cm3. Which of these objects has a greater density? 5. m=?, a=10cm, ρ=800kg/m3 (wood block)

Floating and sinking When a piece of wood is placed in water the wood floats. This is due to the difference in the densities of the wood and the water. When two substances, such as a solid and a liquid or a liquid and a liquid, are put together the less dense substance floats above the denser substance.

Archimedes and the crown
Archimedes is a Greek mathematician, lived in Syracuse (Sicily). He made important discoveries, about the crown from King of Syracuse. Archimedes put the crown in container of water and measured the rise in level. Then he did the same with an equal mass of pure gold. The rise in level was different. So the crown could not have been pure gold. He measured the volume of irregular solid.

To do: Read Fined some example To repeat