Question

why does a screw jack works on the principle of a screw

Answer 1

A screw is like an inclined plane spiraled on a cylindrical rod. We know an inclined plane can be used to lift objects. In the same way a screw when pressed and rotated can be moved up or down. In a screw jack the screw is rotated and the metal part resting on the thread (spiraled inclined plane on the cylindrical rod) moves up or down. Thus, a screw jack works on the principle of a screw. However, in screw jack the screw only rotates but doesn’t move up or down.