Science & Technology in Action

2nd Edition

Buoyancy and How Ships Float

IMDO Ireland

This lesson explains why some things float and others sink, what buoyancy is all about, and how ships are made stable. Significant developments in the history of navigation are also outlined.
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Lesson excerpt

People have been using a great variety of boats and ships for thousands of years and during that time they developed ever greater expertise in boat design, loading and navigation – mainly through trial and error. 

As they became more adventurous they built bigger ships that could carry more cargo and could travel faster and further. With these developments there came the need for more accurate methods of determining distance, speed and position at sea. Although boat-building is still largely an art in many cultures modern ship design is more dependent on the application of scientific principles.

Why do some objects float and others sink?
Is it true that heavy things sink and light things float? At first sight this seems reasonable but on closer examination it is found to be inaccurate. If items having the same weight but made of different materials (e.g. wood, stone, wax, copper, steel etc.) are placed in water some will sink and others will float. The ‘sinkers’ are the same weight as the ‘floaters’ so what distinguishes them? In this case the sinkers will be found to be the physically smaller items – that is, smaller in volume. So we could say that the sinkers are heavier for their size than the floaters.

Alternatively if we had a selection of items that were all the same physical size (e.g. 1 cm3) we would find again that some would float and others would sink; in this case the sinkers would be heavier that the floaters. So whether something floats or sinks depends not just on its weight or its volume but on both. Materials or bodies that are more than 1 gram per cubic centimetre will sink in fresh water, while those that are less than 1 gram per cubic centimetre will float. 

This relationship between mass and volume is what the concept ofdensity describes. In simple terms, dense materials are relatively heavy for their size. 
The density of fresh water is 1 g/cm3 or 1 kg/dm3 (one kilogram per cubic decimetre). The density of wood is around 0.8 g/cm3; this is less than the density of water and so it floats. The density of iron is about 8 g/cm3 and so it sinks in water.

The density of salty water can be a much as 10% greater than that of fresh water – i.e. up to 1.1 g/cm3. An egg will sink in fresh water but it will float in very salty water; the density of the egg is greater than the density of fresh water but less than the density of the salty water.

True or False?

  1. In general heavy things sink and light things float.
  2. Density is how big an object is.
  3. The units of density are: kg/m3, g/cm3 or kg/dm3.
  4. Buoyancy is the force that pushes things under water.
  5. The buoyancy force on a football under water is about 20 N (the weight of 2 kg).
  6. The pressure at a depth of 20 m in fresh water is three time atmospheric pressure.
  7. When a body is fl oating it appears to lose some of its weight.
  8. The pressure in a liquid depends only on its density.