Liquids do not compress
Liquids are not compressed when a force is applied to them. Because of this, a pressure applied at one place in a liquid can be transmitted to another place in it.
Put a gas in a syringe and push the piston and it moves in — the gas is squeezed. Fill the syringe with water, close the end and push, and the piston hardly moves at all. The water has nowhere to be squeezed into.
Precisely because there is no room to squeeze
A pressure applied at one place travels through the liquid undiminished and arrives at every other place at the same size. This is not some odd property of liquids — it follows from their not being compressible.
The machines built on this
Machines built on this phenomenon are called hydraulic machines. They let us apply a small force and obtain a large one.
- the hoist used to lift vehicles
- the hydraulic jack used to raise one side of a car
- the braking system of motor vehicles
Over the next four pages we look at how each of these works — and all of them rest on this one idea.