When a car tire breaks down during driving, the driver will stop to change the tire, which requires lifting the car. A small car weighs about 1 ton, and a large truck weighs several tons. It's absolutely impossible for humans to lift them. However, the driver will take out a jack from the car's toolbox and easily lift the car. How can such a small jack have such great power? How does it work?
To lift a heavy object vertically, the force applied must be greater than or equal to the weight of the object. But pushing a heavy object up a slope requires much less effort. The smaller the slope angle, the less friction there is between the slope and the object, and the less force is needed. Therefore, when loading heavy objects onto trucks, inclined ramps are often set up at the back of the trucks, making it much easier to push the objects onto the trucks. But if you let go of the object when pushing it up the slope, would it slide down the slope? Wouldn't all your efforts be in vain? If the slope is relatively steep, this is entirely possible. But if the slope angle is relatively small, the component of gravity along the slope is equal to the friction between the object and the slope, so the object won't slide down without any pushing force. This phenomenon is called "self-locking". This principle is also commonly applied in life, such as using wedges to support the roof beams in tunnel construction. After wedges are inserted under the roof beams, as long as the self-locking condition is met, the roof beams won't slide down the slope of the wedges.
Building a slope requires a large space, which is often not available in real life or on construction sites. We can roll up the slope around a vertical axis, forming a spiral surface, which is equivalent to building a slope in a narrow space, just like the spiral staircases in buildings. The screw threads we see every day are just such spiral surfaces. Of course, the helix angle of the screw threads also needs to meet the self-locking condition. When a bolt rotates in a nut, it's actually an object sliding on a slope, but the pushing force is replaced by a torque couple. The structure of a jack is equivalent to a rolled-up slope. Its base is a large nut, and the lifting rod is a bolt. The lifting rod is connected to a long handle, which also utilizes the lever principle in physics. When we rotate the handle at both ends with a not-too-large torque couple, a large lifting force will be generated at the top, easily lifting the car up.
There are various forms of jacks, but the working principle is basically the same. There is also a hydraulic jack that uses the principle of hydraulic transmission.