Hydraulic Machine

Definition of Hydraulic Machine

The machine in which force is transmitted by liquids under pressure is known as hydraulic machine.
Example: Hydraulic Brakes, Hydraulic Jack

Hydraulic Brakes

The brakes used for car are hydraulic brakes.

Hydraulic Brakes

  • The above diagram describes the working principle of Hydraulic brake.
  • When the brake pedal is pressed, a piston force breaks fluid from one cylinder along a connecting pipe to another cylinder.
  • There, the fluid pushes on another piston. This pushes a brake pad against a metal disc attached to the rotating wheel of the car.
  • The friction slows the wheel.

More About Hydraulic Machines:

  • Liquids are generally incompressible (they cannot be compressed).
  • When liquid is trapped (closed in a container) and pressure is exerted, this pressure is transmitted to all parts of the liquid.
  • The force (or pressure) is exerted on small area, but it results on a large area with much force.
  • Thus the small force applied on a car brake results large force on 4 wheels.

Worked Example

The following system contains fluid

Definition of Hydraulic Machine

A downward force of 20 N is applied on the piston X.
What will be the upward pressure exerted by the liquid at Y.
    A. 4 N/Cm2
    B. 1 N/Cm2
    C. 20 N/Cm2
    D. 40 N/Cm2
    Correct answer is "B".


    Step 1: Pressure = force /area. Same pressure acts on both the pistons.
    Step 2: Force = 20N, Area = 20Cm2
    Step 3: Pressure = 20/20 = 1 N/Cm2

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