mechanical · 30 minutes · Official OETT Aligned
Liquid Pressure & Hydraulics
How liquid transfers a push
What to know
- pressure: force spread over an area
- piston: a sliding part that pushes liquid
- piston size and force
- how far each piston moves
Hydraulic systems use liquid pressure to turn a smaller push into a larger lifting force.
The core rule
Pressure = force ÷ area. Output force = pressure × output piston area.
Hydraulics use trapped liquid to carry pressure between pistons. A piston is a sliding part that pushes against the liquid. In a simple system without losses, a larger output piston gives more force at the same pressure. It moves a shorter distance because both pistons move the same amount of liquid. A narrow opening makes liquid harder to push through.
Worked examples
Example 1: easy
30 lb on 2 in² produces 15 psi.
- Divide 30 lb of force by 2 square inches of piston area.
- 30 ÷ 2 = 15 psi. Psi means pounds per square inch.
The liquid carries this pressure through the simple system.
Example 2: medium
At 15 psi, a 10 in² piston produces 150 lb.
- The output piston has 15 psi acting over 10 square inches.
- Multiply: 15 × 10 = 150 lb of force.
The same pressure gives more force over a larger area.
Example 3: hard
If output area triples, ideal force triples while output travel becomes one-third for equal volume.
- Keep the pressure the same and triple the output piston area. Its force triples.
- For the same amount of liquid moved, the output piston travels one-third as far as before.
Extra force comes with less travel.
Common mistakes
A hydraulic system gives more force and more travel at the same time.
The larger piston provides more force but moves a shorter distance.
Put it into practice
Use guided explanations, check your answers, and try a new variation.