What is mechanical advantage in a simple machine?

Mechanical advantage is the force-amplifying effectiveness of a simple machine, such as a lever, inclined plane, wedge, wheel and axle, or pulley system.1

It is defined as the ratio of the output force (the load) to the input force (the effort). In an ideal system without friction, the theoretical mechanical advantage represents the ratio of the force performing useful work to the force applied. In practice, however, the actual mechanical advantage is always lower than the theoretical value due to the presence of friction, which causes energy loss.21

A key principle of simple machines is that what is gained in force is typically lost in motion; the velocity ratio—the ratio of input motion to output motion—is generally equal to the mechanical advantage when efficiency is high.2