A pulley is a simple machine consisting of a wheel and a groove, used to change the direction of a force or to reduce the effort required to lift a heavy object. These devices operate on the principle of mechanical advantage, utilizing the tension in a rope or cable passing over the wheel. Depending on whether the pulley is fixed or movable, it can alter the necessary effort force or the distance over which the force must be applied. Systems involving multiple pulleys can combine these effects, creating compound machines that efficiently transmit force along fixed lines or around moving axes for various engineering applications.