A hydraulic car lift uses a small piston to push oil through a pipe connected to a large piston beneath a car. Explain why the pressure exerted by the small piston is transmitted equally in all directions through the oil, and explain how this allows the large piston to exert a greater force on the car than the force applied to the small piston.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A hydraulic car lift consists of two pistons of different areas connected by oil-filled pipes. A mechanic applies a force to a small piston of area 0.002 m² to raise a car resting on a large piston of area 0.10 m².
Model answer (4 marks)
Pressure in a fluid acts equally in all directions because the molecules of the liquid transmit the force through collisions; the pressure at any point is the same in every direction.
The same pressure that acts on the small piston is transmitted through the oil to the large piston.
Force = pressure × area, so for a given pressure the force is proportional to the area.
Because the large piston has a greater area than the small piston, the force it exerts on the car is larger than the force applied by the mechanic.
The same pressure that acts on the small piston is transmitted through the oil to the large piston.
Force = pressure × area, so for a given pressure the force is proportional to the area.
Because the large piston has a greater area than the small piston, the force it exerts on the car is larger than the force applied by the mechanic.
Examiner tips
- Use the phrase "pressure acts equally in all directions" to show understanding of Pascal’s principle.
- Show the equation F=PA and explain that the same P acts on both pistons.
- State that the large piston’s area is greater, giving a larger force.
- Keep the answer concise – 4 points can be covered in 4 short sentences.
Common mistakes
- Confusing pressure with force – writing that the small piston exerts a larger force. Forgetting to mention that the same pressure acts on both pistons. Using the wrong equation (e.g., F=mv) or omitting the area term.
Mark scheme (4 marks)
- Pressure in a fluid (liquid) acts equally in all directions / is transmitted equally throughout the oil
- The same pressure acts on the large piston as on the small piston
- Force = pressure × area, so a greater area produces a greater force (for the same pressure)
- The large piston has a greater area than the small piston, so the upward force on the car is greater than the force applied by the mechanic
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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