Understanding density is the key to understanding why objects float on water. If the weight of water displaced by a submerged object is less than the weight of the water displaced, then it will float.
If you don’t understand this then read on. By the time you have finished reading this article it will all make sense.
Density is a relatively simple concept, technically it is the mass of a substance per unit of volume but even that explanation doesn’t make a lot of sense if you are unfamiliar with the concept.
It is probably easier to understand by taking an example that we can relate to on an every day basis. If a brick is placed on the surface of a body of water, it’s a safe assumption to think that it will immediately sink, as long as the water is deep enough. Conversely, if a piece of wood is placed on the same surface, it’s a safe assumption to think that it will float. So why is this?
In order to answer this question, let’s look at the density of each of the materials in question. Density is a property of a material that tells us how heavy it is per unit of volume. And it can be calculated with the following formula
D = M/V
Where D is density, M is mass and V is volume.
Using this formula, if we take 1000cm3 of water and weigh it, we would see that it weighs 1000g. Using the above formula we can see that the density of water is
1000g/1000cm3 = 1g/cm3or in words it is 1g per cm3.
If we want to know what will float on water then it makes sense that it should be something that is ‘lighter’ than water. That is for each unit of volume it should have less mass than the same volume of water. In other words it should be less dense.
Now let’s go back to the brick. We can see whether a brick will float in water or not by working out its density and comparing that to the density of water. An average house brick has the following dimensions.
Width = 20cm
Height = 4cm
Depth = 10cm
The total volume of the brick would be 20cm x 4cm x 10cm = 800cm3. (The small 3 in superscript tells us that we are dealing with 3 dimensions or volume.) To work out the density then, we simply need to know the mass of the brick, which is around 2,500g. Using the same formula D=M/V we can work out the density of the brick which is;
2,500g/800cm3 = 3.125 g/cm3
Compare the density of the brick to the density of water at 1g/cm3 and immediately you can see it is roughly three times more dense, so it will sink in water.
We can use exactly the same method to calculate the density of wood. A block of pine in the same dimensions as the brick would have a mass of around 400g. If we work out the density we would find it is 0.5g/cm3. This is less than the density of water so the pine will float.
Of course you could just throw the brick and the wood into a pond and see what happens but how would you get the brick back if you needed it?
