i About this experiment — click to learn the physics ▼
What you're looking at
Two pistons sit on the same trapped body of fluid — a small one on the left and a large one on the right, joined at the bottom. Push down on the small piston and the large one pushes up, lifting a load far heavier than the force you applied. That's a hydraulic press, and Pascal's principle is why it works.
Pascal's principle
A pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid and the walls of its container. So the pressure you make under the small piston appears, unchanged, under the big one:
Force multiplication
Because the pressure is the same but the big piston has more area for it to push on, the output force is multiplied by the ratio of areas:
With a 5× area ratio, a 100 N push becomes a 500 N lift. This is how a hydraulic car jack or the brakes on a car turn a gentle effort into a powerful force.
No free lunch
Energy is still conserved — you don't get the force for nothing. The fluid pushed out from under the small piston has to reappear under the big one, so the big piston moves less by exactly the same factor:
Push the small piston down 50 cm and, at 5×, the load rises just 10 cm. Force × distance — the work — comes out the same on both sides.
Things to try
Set a heavy load, then raise the area ratio until the output force finally exceeds it and it lifts. Notice the big piston's stroke shrinks as the ratio grows — the price of the extra force.