A bar or lever is a rigid structure, which is based on an axis. It is used to lift weights and to reduce or increase the intensity of forces or movements. In this machine simple act two forces: the driving force or power, and the resistance of the body being overcome. A pressure at any point of the lever may be balanced by another force located at another point. There are three types of levers. The first genre is the fulcrum located between the driving force and resistance (balance, scissors), in the second type, resistance is exerted between the fulcrum and power (trolley) and those of third-order driving force is applied between the resistance and the fulcrum (hammer, pedal grinder). The movements of certain body parts such as legs, arms, head or hands are natural examples of the actions of a lever, where the driving force is applied through certain muscles
A lever is in equilibrium when the sum of the forces exerted on the fulcrum is zero, ie when the product of the driving force by the distance from the fulcrum is equal and opposite to the product of resistance by distance at the same point of support.
The shear is a special case of first-class lever. Each of the two pieces that form act as levers, which have a common fulcrum.