A simple mechanical machine is a device that changes the direction or/and magnitude of a force. A simple machine can be defined as the most simple mechanism which can provide mechanical advantage.

Simple Machines

The forces acting on simple mechanical machines can be divided into:

Resistance force \((R)\)
The strength you want to win, or balance, with the aid of a machine is called resistance.
Effort force \((\Gamma)\)
The force that is necessary to apply on the machine, to give the desired effect, is known as effort or driving force.

For a mechanical machine in equilibrium, the mechanical advantage is defined as the ratio of the effort force and resistant. Mathematically it is:$$VM = \frac{\Gamma}{R}.$$

Levers

The lever is a rigid bar which can rotate around an axis in which it is supported. The rotation axis of a lever is called fulcrum.

First Class Lever
They are levers where the fixed point is between the resistant force and effort.
Examples: scales, scissors and pliers.
Second Class Lever
They are levers where the resisting force is between the effort force and the fixed point.
Examples: wheelbarrow, nutcracker and garlic press.
Third Class Lever
They are levers where the levers' effort force lies between the resistant force and the fixed point.
Examples: tongs, ice tongs, fishing rod and car accelerator.

The equilibrium condition for any lever is: $$ R \times B = \Gamma \times b,$$ where \(B\) is the force of the resilient arm and \(b\) is the effort arm of force.

Pulleys

The pulley is used to transfer force and motion. A pulley is composed of a rigid material wheel which rotates on a shaft driven by a belt or rope.

Fixed Pulley
Balance condition: $$\Gamma = R$$ .
Movable Pulley
Balance condition: $$\Gamma = \frac{R}{2}$$ .
Compound Pulley
It is the combination of pulleys with one fixed. In the case of \(n\) movable pulleys, the driving force is: $$\Gamma = \frac{R}{2^n} $$ .

Inclined Plane

It is a rigid plane, in this given example frictionless and inclined of an angle \(\theta\) (see drawings above). The equilibrium condition for this machine is: $$ \Gamma = P sen(\theta). $$ Examples of machines that use the principle of the inclined plane: screw, jack screw and wedge.