In this topic you will learn about how other simple machines work and how they can be combined into compound machines.

A ramp is an example of an inclined plane. An inclined plane is a straight, slanted surface that is not moved when it is used. Without any moving parts, inclined planes can make it easier to do work because they multiply the effort force. For inclined planes, dividing the length of the incline by its height gives the mechanical advantage (assuming no friction).

A screw is an inclined plane wrapped around a central bar. A screw is actually a type of simple machine that multiplies effort. The mechanical advantage of screws can be very high. The spiral ridges are the threads of the screw. The distance from thread to thread is called the pitch. The distance around the head of the screw is the distance the effort moves. The distance through which the output moves is equal to the pitch of the screw. The mechanical advantage of a screw is the distance around the head of the screw divided by the pitch of the screw.

A wedge is one inclined plane or a combination of two inclined planes joined back-to-back. Unlike an inclined plane, a wedge must be moved by an effort force. A wedge can change the direction of an effort and increase its strength. Knife blades, ax heads, chisels, and other cutting tools are actually wedges.

Scissors are a relatively basic device. However, scissors are made of more than one simple machine. When we make a device that is a combination of two or more simple machines, a compound machine results. A screwdriver being used to turn a screw is an example of a compound machine. The screwdriver itself is a wheel and axle. A screw is another simple machine. The screwdriver and the screw combine to make a compound machine.

A machine changes work done by an effort force into work done by a machine's output force. In a machine with no friction, the work input equals the work output. However, no machine truly runs without friction. When there is friction, some of the work input is changed into heat energy. This means not all of the work input becomes work output. As a result friction makes the work put into a machine greater than the work done by the machine. The ratio of work done by a machine compared to the work put into the machine is called efficiency.