Definition

A simple machine is a tool with few or no moving parts that makes a job easier by letting you use a smaller push, or push in a more convenient direction. It is a device that changes the magnitude or direction of a force, with six classic types: the lever, pulley, inclined plane, wedge, screw, and wheel and axle. Mechanical advantage is the ratio of output force to input force, and a simple machine cannot reduce the work required; it only spreads it over a longer distance or changes its direction. Precisely, an input force $F_{\text{in}}$ acting through distance $d_{\text{in}}$ becomes an output force $F_{\text{out}}$ through $d_{\text{out}}$, with ideal mechanical advantage $\text{IMA} = d_{\text{in}}/d_{\text{out}}$ and actual mechanical advantage $\text{AMA} = F_{\text{out}}/F_{\text{in}}$. Efficiency is $\eta = W_{\text{out}}/W_{\text{in}} = \text{AMA}/\text{IMA}$, always below $1$ because of friction, and all six types reduce to two families, the lever family (lever, pulley, wheel and axle) and the inclined-plane family (ramp, wedge, screw).