Simple machines
Simple machines are devices that change the size or direction of a force and make work easier: lever, pulley, inclined plane, wedge and windlass. A lever is a rigid body that can turn about a fixed support (scissors, pliers, a wheelbarrow). A fixed pulley only changes the direction of the force; a movable pulley gives a 2-fold gain in force. An inclined plane lets you raise a load with less force than lifting it straight up: the longer and gentler the ramp, the smaller the force (without friction F = P × height ÷ length). A wedge is two inclined planes joined (axe, knife, nail). A windlass raises a load with a turning handle (a well windlass). The “golden rule of mechanics”: whatever the gain in force, the loss in distance is the same, so there is no gain in work.
Inclined plane: lean a long board or thick cardboard on books and let a toy car roll down. With two different slopes compare the rubber-band stretch needed to pull the car up. Safety: set the board and books firmly so they cannot fall on feet; avoid boards with sharp splinters.