The Wedge

The wedge is a kind of inclined plane. The difference is that an inclined plane stays in one place, but a wedge moves. One common use of a wedge is for splitting logs to make firewood. Wedges are sometimes used under doors to prevent them from blowing shut. The Egyptians may have used wedges to split the rocks that were used in building the pyramids. By hammering several wedges into a crack between two layers of rock, the rock could be weakened. Eventually the rock would split. 

The Screw

The screw is similar to the inclined plane and the wedge. It is an inclined plane that is rolled up. You can see this if you cut a right-angle triangle of paper, darken the long side, and roll it around a pencil as shown in the drawing. The pattern that results is much like the thread on a screw. If the inclined plane has a gentle slope, the thread will be fine. If the slope is steep, the thread will be coarse. You can follow the edge of the paper around the pencil with your finger and see how it makes a spiral. Screws are used to join pieces of wood or metal. When you use a wood screw, you apply a turning force to the screwdriver with your hand and wrist. As the screw turns, the threads pull the screw into the wood. In other words, a screw changes a turning force into a pulling force. Threads have many uses. The lids of jars and pop bottles are often threaded so that they can be closed tightly. Bolts and nuts depend on threads for doing their task of holding things together.