Compound machine: A machine consisting of two or more simple machines.
Effort force: The force applied to a machine.
Friction: A force caused by the movement of an object through liquid, gas, or against a second object that works to oppose the first object's movement.
Mechanical advantage: A mathematical measure of the amount by which a machine magnifies the force put into the machine.
Resistance force: The force exerted by a machine.
Simple machines include the lever, the inclined plane, the wedge, the pulley, the wheel and axle, and the screw. Combine these and you can create almost anything to make work easier.
A machine is an object or mechanical device that receives an input amount of work and transfers the energy to an output amount of work. For an ideal machine, the input work and output work are always the same. Remember that work is force times distance; even though the work input and output are equal, the input force does not necessarily equal the output force, nor does the input distance necessarily equal the output distance.
Machines can be incredibly complex (think of robots or automobiles), or very simple, such as a can opener. A simple machine is a mechanical device that changes the magnitude or direction of the force. There are six simple machines that were first identified by Renaissance scientists: lever, pulley, inclined plane, screw, wedge, and wheel and axle. These six simple machines can be combined together to form compound machines.
We use simple machines because they give us a mechanical advantage. Mechanical advantage is a measurement of the force amplification of a machine. In ideal machines, where there is no friction and the input work and output work are the same,
(Effort Force)(Effort Distance)=(Resistance Force)(Resistance Distance)
The effort is the work that you do. It is the amount of force you use times the distance over which you use it. The resistance is the work done on the object you are trying to move. Often, the resistance force is the force of gravity, and the resistance distance is how far you move the object.
The ideal mechanical advantage of a simple machine is the ratio between the distances:
IMA=effort distance resistance distance
Again, the IMA assumes that there is no friction. In reality, the mechanical advantage is limited by friction; you must overcome the frictional forces in addition to the resistance force. Therefore, the actual mechanical advantage is the ratio of the forces:
AMA=resistance force effort force
When simple machines are combined to form compound machines, the product of each simple machine's IMA gives the compound machine's IMA.