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Hydraulic brakes and presses

These are useful ways that the pressure in a liquid can be used. Many cars, and even some mountain bikes, have hydraulic brakes where a small force on a small cylinder can give a large force at the other end of a larger cylinder. (Figure 1)

The important fact to remember is that the pressure throughout the liquid must be the same and so the pressure in the bigger cylinder (F/A) must be equal to the pressure in the little cylinder (f/a).


F/A = f/a and so F = fA/a
‘Output force’ = force on small piston x [area of large piston/area of small piston]

This means that because the larger cylinder has a much bigger cross sectional area than the smaller cylinder the force on this cylinder (F) is much greater than that on the small cylinder – it acts as a sort of force magnifier!The great power of hydraulics can be seen if you watch a skip being hauled up onto a lorry by a hydraulic lift.

The hydraulic press and jack

Another useful application of the force magnifying action of liquid pressure is the hydraulic jack or hydraulic press (Figure 2).

The pressure exerted by the force on the small piston on the left of the diagram is transmitted through the liquid. This means that large objects can be lifted by the cylinder on the right or large forces can be exerted on object held beneath the top plate as shown by the hydraulic press in Figure 2.

Example problem
A force of 25 N is applied to the smaller cylinder of a hydraulic press. If the area of this cylinder is 8 cm2 and the area of the large cylinder is 100 cm2 what is the force exerted by the large cylinder?

Force (F) = fxA/a = 25 x 100/8 = 312.5 N

The hydraulic brake

© Keith Gibbs 2010