A hydraulic jack uses a liquid trapped in connected cylinders. Pressure applied to the liquid is transmitted through it. Pressure equals force divided by area. A small force on a small piston can create a pressure that acts on a much larger piston. Because the larger piston has a greater area, the same pressure produces a larger output force. The trade-off is distance: the small piston must move farther than the large piston rises. The jack does not create energy for free; it exchanges a smaller force over a larger distance for a larger force over a smaller distance.

Engineers use this relationship when choosing shapes and contact areas. Ships, hydraulic systems, tyres, foundations and cutting tools may look unrelated, but each is designed around how forces are spread through a material or fluid. Similar principles appear in nature too, from broad animal feet on soft ground to floating ice changing the conditions at the surface of a lake.

Pressure is the force acting on each square metre of area, so the same force can have very different effects depending on where it is applied. In liquids, pressure is transmitted through the fluid; this is why a small force on a small piston can produce a much larger force on a larger piston. The trade-off is distance: the larger piston moves a shorter distance, so the machine does not create energy from nowhere.