Science Puzzle
How Can One Hand Lift a Car?
A family car weighs well over a tonne. Nobody could lift one corner of it with their bare hands. Yet with a small hydraulic jack, a person can raise a car off the ground by pumping a handle with one hand.
The jack has no motor and no battery. Where does all that lifting force come from?
The Answer
The jack trades distance for force, using a trapped liquid to multiply your push.
Inside the jack, oil fills a narrow cylinder and a wide one, joined by a pipe. Oil hardly squashes at all, so when you push down on the small piston, the extra pressure spreads evenly through all the oil. Every square centimetre of the big piston feels the same push as every square centimetre of the small one. If the big piston has 50 times the area, it pushes up with 50 times the force.
There's a catch, and it's the clue in the hint. To raise the big piston by 1 centimetre, you have to push 50 centimetres' worth of oil out of the small cylinder. Force goes up, distance goes down, and the work you do stays the same. That's why you pump the handle many times for the car to rise a little. A one-way valve lets fresh oil into the small cylinder on each upstroke without letting the big piston sink back. The long handle works as a lever, which multiplies your force again.
Blaise Pascal described how pressure spreads through a liquid in the 1600s, and Joseph Bramah patented a hydraulic press in 1795. The same idea works car brakes, diggers and the controls of large aircraft.
The principle: Pressure in a liquid. Pressure on a trapped liquid spreads equally throughout it, so a small force on a small piston becomes a large force on a large one, at the cost of moving further.