The efficiency of a single movable pulley system is not 100% this is because (i)The friction of the pulley bearing is not zero , (ii)The weight of the pulley and string is not zero. b) The efficiency of a movable pulley is always less than 100%. ... (a) In a single fixed pulley, some effort is wasted in overcoming friction between the strings and the grooves of the pulley; so the effort needed is greater than the load and hence the mechanical advantage is less than the velocity ratio. When you pull either up or down on the rope, the wheel will turn. This makes moving the object easier, because pulling a rope downward, for instance, is easier than lifting a heavy object upward. Of course, a single pulley will not affect the amount of work necessary to move an object very much.