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Grades 8 and 10 Intermediate About 20 min Version 1 Awaiting review Premium

Pascal's principle: the hydraulic lift

Liquids cannot be squashed. Pascal's principle says that a pressure applied to a liquid in a closed container is passed on equally to every part of the liquid. In a hydraulic lift, a small force on a small piston makes a pressure P = F₁ ÷ A₁. The same pressure pushes up on the large piston, giving a force F₂ = P × A₂, which is bigger because A₂ is bigger. But the large piston moves a shorter distance, so no energy is gained. Car lifts, hydraulic brakes and JCB diggers work this way.

What you will learn

  • State Pascal's principle.
  • Calculate the output force of a hydraulic machine using F₂ = F₁ × A₂ ÷ A₁.
  • Explain why the large piston moves a shorter distance.

Before you start

  • Calculate pressure = force ÷ area.

Materials

  • Syringes of different sizes joined by a tube filled with water (a model)

Equipment

  • Hydraulic lift model with interchangeable pistons

Procedure

  1. Push with 20 N on the 1 cm² piston with the 100 cm² piston on the other side. Record whether the car lifts.
  2. Change the areas and the effort until the 4000 N car is lifted, and record it.
  3. Compare the pressure under each piston and how far each piston moves.

How this simulation is simplified

  • The oil cannot be compressed and there is no friction at the pistons.
  • The car weighs 4000 N, and the small piston is pushed down 10 cm each time.