Definition Of 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