Simple machines: pulley, inclined plane, screw, wedge and windlass
Simple machines serve to change a force or its direction. A fixed pulley (a wheel with a fixed axle) gives no gain in force, it only changes the direction: we pull down to lift the load up. A movable pulley (one that rises with the load) gives a 2-fold gain in force but loses 2-fold in distance. A system of several pulleys (polyspast) gives a greater gain. On an inclined plane the force needed to lift a load is, ideally, F = m · g · h : s, where h is the lifting height and s the length of the plane; the longer and gentler the ramp, the smaller the force. The screw and jack, and the wedge (axe, knife, nail) are kinds of inclined plane. A windlass (a well winch) is a cylinder with a handle: it gives a gain in force of R : r, where R is the radius of the handle’s circle and r the radius of the cylinder. A machine that gives a gain in force by some factor loses the same factor in distance.
Observation: find a fixed pulley (flag-raising block), a well winch, a screw cap and a ramp, and discuss what gain each gives. Safety: do not lift heavy loads, keep hands out of mechanisms; work with pulleys and windlasses only with an adult.