What is a zip
A zip consists of two sets of interlocking teeth. As shown in figure 8.2.6, the zipper uses 3 wedges to force these teeth together when doing the zip up.
![Picture](/uploads/2/6/1/2/26122089/8131835.jpg?223)
Figure 8.2.6
zip consists of a zipper track, containing two rows of teeth. The zipper is closed when the hooks in the teeth on one side of the zipper are slotted into hollows of teeth on the other side. The zipper slide is a collection of wedges. A central wedge unlatches the teeth as the zip is undone and the side wedges force the teeth together when the zip is done up.
What is a screw
A screw is a spiral inclined plane. (figure 8.2.7) It is a simple machine that is designed to cut through another substance. e.g wood screw cut through timber, boat propellers cut through water and aircraft propellers cut through air.
Details about screws
The spiral ramp along the screw enables you to drive the screw into the wood with much less force than it would be needed if you hammered it directly. This occurs because, the screw moved a greater distance through the timber. The spiral inclined plane around the screw is called a thread. As figure 8.2.8 shows, the closer the turns of the threads are, the gentler the slope or pitch of the inclined plane will be, and the force will be magnified.