Density
In everyday life, we are often interested in the weight of objects. I can tell by lifting a piece of wood, whether it is lighter or heavier than a coin.
Quite often, we also like to know whether a body floats or sinks in water. It is tempting to think that a heavy body sinks, and a light body float. A few quick experiments with a basin of water would convince us that this is not true. Drop a small coin into the water, and it sinks. Put a large piece of wood on the water, and it floats. This happens even if the wood is heavier than the coin.
A body sinks or floats not because of it is heavier or lighter. It is the density that is important here. Density is the mass per unit volume of the body. Even if the large piece of wood is heavier than the small coin, the mass of the wood is smaller than the mass of the coin if we just look at the same volume. We say that the density of the wood is less than the density of the coin.
We can find density ρ using the formula ρ = m/V, where m is the mass of the body and V the volume.
Exercise. A piece of wood has a mass of 8 g and a volume of 10 cm3. Find its density. Answer. ρ = m/V = 8/10 = 0.8 g/cm3.
Exercise.A piece of wood has a mass of 1 g and a volume of 0.2 cm3. Find its density. Answer. ρ = m/V = 1/0.2 = 5 g/cm3.
From these exercises, we can see that if we compare
1 cm3 of the wood with 1 cm3
of the coin, the wood is lighter.
Copyright 2010 by Kai Hock. All rights reserved.
Last updated: 5 October 2010.