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A machine that uses liquid to transmit a force is called a hydraulic system. The pressure in the liquid is the same everywhere in the system. The simplest type of hydraulic machine is made of two pistons connected by a liquid-filled pipe. If one piston is pushed, the force is transmitted through the liquid and the other piston moves. Hydraulic brakes on cars work like this.

Liquids are used in hydraulic systems because they are more difficult to compress than gases, so are better at transmitting forces - when a force is applied at one end of the system, it is easy transferred through the system.

Calculating the force on a piston

A hydraulic car jack is used to lift a vehicle. The area of piston A holding the car is 40 m² and the area of piston B, which the mechanic presses, is 0.5 m². The mechanic exerts a force of 100 N on her piston. What is the force exerted on the car by piston B? (Remember that the pressure is the same in the entire system.)

First calculate the pressure on the surface of piston A:

On piston A, a force of 100 N is exerted over an area of 0.5 m².

This is the pressure of the system.

Then calculate the force at piston B. First rearrange the equation to find force normal to surface area:

Force normal to a surface area = pressure × area of that surface

Force on piston B = 200 × 40

Force on piston B = 8,000 N