A liquid cannot be squashed much, and any pressure applied to an enclosed liquid is transmitted undiminished to every part of it. This is Pascal's principle.
A hydraulic jack has two connected cylinders of different sizes filled with oil. You push down on the small piston with a modest force, which creates a pressure in the oil. That same pressure acts on the much larger area of the big piston. Since force is pressure times area, the larger piston feels a much greater force, enough to lift a car, while you only supply a small effort over many strokes.
The trade-off is distance: the small piston must move a long way to raise the big piston a little, so no energy is created, only force is multiplied. The same idea drives hydraulic brakes and excavator arms.
In SPM you should apply Pascal's principle and the relation between force, pressure and area.
Common misconceptions
- A hydraulic jack creates extra energy to lift the car -> It multiplies force, not energy; the small piston moves much further, so work is conserved.
- The oil could be replaced by air with no difference -> A gas compresses easily and would absorb the effort; a liquid transmits the pressure fully.
- The big piston pushes harder because oil is thicker there -> The pressure is the same everywhere; the bigger force comes from the larger area.
The physics behind it
A hydraulic jack works by Pascal's principle, which states that pressure applied to an enclosed fluid is transmitted equally in all directions throughout the fluid. Pressure is force per unit area, P = F/A, measured in pascals (Pa), where 1 Pa = 1 N m⁻².
The jack has two pistons of different area joined by oil, and because the pressure is the same everywhere in the oil, a small force on the small piston produces a large force on the large piston: F₁/A₁ = F₂/A₂.
Suppose the small piston has area A₁ = 2 cm² = 2 × 10⁻⁴ m² and you push with F₁ = 100 N. The pressure created is P = F₁/A₁ = 100 N ÷ 2 × 10⁻⁴ m² = 5 × 10⁵ Pa. If the large piston has area A₂ = 100 cm² = 1 × 10⁻² m², the force it delivers is F₂ = P × A₂ = 5 × 10⁵ Pa × 1 × 10⁻² m² = 5000 N.
A push of 100 N lifts a load of 5000 N, because the large piston has fifty times the area of the small one.
See it in daily life
The same principle is at work every time a mechanic raises a car in a workshop. A few firm strokes on a slim handle, which drives a narrow piston, lift a load far heavier than any person could raise by hand.
You gain force, but not energy for nothing: the small piston must move a long way to raise the large piston a little, so the work you put in equals the work you get out, minus friction.
A car's foot brake uses hydraulics too. Gentle pressure on the brake pedal is transmitted through brake fluid to the much larger pistons at each wheel, pressing the pads onto the discs with great force.
A barber's chair and the tipping bed of a lorry rise the same way.
In each case an incompressible liquid, usually oil, carries the pressure, because a liquid barely changes volume when squeezed, unlike a gas, which would simply compress and waste the effort.
How this comes up in SPM
In Paper 2 this is examined under command words such as State, Explain and Calculate, within the pressure chapter. You are often asked to state Pascal's principle precisely, then explain how a hydraulic system multiplies force, referring to equal pressure being transmitted through the fluid.
Calculations usually give you two piston areas and one force and ask for the other force using F₁/A₁ = F₂/A₂, so convert areas to square metres and forces to newtons, and keep the pressure consistent. A frequent follow-up asks why the liquid must be incompressible, or why a gas would not work as well.
The topic sits beside pressure in liquids, atmospheric pressure and the gas laws, so be ready to define pressure as P = F/A with units of pascals. Watch for questions that also test energy conservation, asking you to compare the distances moved by the two pistons and confirm that the work input equals the work output when friction is ignored.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)