The pulley is a device that makes it possible to lift a heavy weight with little force. The force required varies according to the number of pulleys employed. One pulley will require a force equal to the weight to be lifted but the force required diminishes proportionally as the pulley number increases.
The model tests several pulley systems. The required force is equal to the weight lifted in the first example, half the weight lifted in the second example and 1/4 in the third.