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Lesson: Chapter 15 — Fluid Pressure and Its Applications

15.3. Transmitting pressure through liquids 3 of 5

The vehicle hoist

The hoist used to lift vehicles at garages and service stations is another device built on the transmission of pressure.

Figure to be added

Figure 15.7 — a hoist used for lifting vehicles; the small piston, the oil joining them and the large piston that raises the car, together with the distances each one moves.

The sequence of events

  1. A force of small magnitude is applied to the small piston.
  2. That produces a pressure in the oil.
  3. The oil transmits that pressure to the large piston.
  4. Because the large piston has a large area, it receives a force equal to the weight of the car.
  5. The car is lifted.

The price paid for the gain in force

As the figure shows, the small piston moves much further than the large one does. Since the liquid cannot be compressed, the volume pushed out from under the small piston must arrive under the large one.

Nothing comes free

Whatever factor the force is multiplied by at the large piston, the distance it moves is divided by that same factor. A hoist gives a gain in force, not a gain in energy.

That is why the small piston has to be pumped down several times to lift a car.