The SI unit for density is kg/m3, but for the size and mass of objects you will come across in school, you will probably use g/cm3.
What happens next?
Physical World Key Words
Forces: Floating and Sinking Learning Intentions
In this topic we are learning to: z Measure and calculate density. z Evaluate the relationship between density and floating bodies. z Illustrate how bodies experience upthrust.
Density
To find out if something will float or sink in water, you need to know its density.
Density is a measure of how much matter is in a substance compared to how much space it takes up.
Density is the ratio of mass to volume. The formula for density is:
Density = Mass (kg) Volume (m3)
Unit of density = kg/m3
A can of Coca-Cola and a can of Diet Coke, both of equal volume, are placed into a large container of water.
1. Predict what happens next, explaining your reasoning. 2. Carry out this test at home and record your observations.
3. Explain why your observations did or did not match your prediction.
Polystyrene
Cork float
Methylated spirits
Corn oil
Plastic building block
Water
Rubber tubing
Sugar syrup
Glass marble
Fig. 17.7.1 Finding water density
The density of pure water is 1 g/cm3. Using appropriate instruments, prove that this is correct.
If a material has a mass of 120 g and a volume of 150 cm3, its density is calculated as:
Density = Mass Volume
Density = 120 150 = 0.8 g/cm3
This material will float in water because it has a density less than water. If its density was greater than the density of water, it would sink.
300 300
Fig. 17.7.2 Different density liquids and materials
