How to Use a Single and Double Pulley System | Sciencing. 3/1/2018 · When you need to lift a heavy weight, a pulley can make the job easier. A pulley is one of the physic's six non-motorized simple machines—in this case, one that reduces either the effort necessary to lift a load or the direction in which the force needs to be applied in order to move the load—or it may do both things