Through a combination of simple machines, mobile cranes are able to gain a mechanical advantage to lift heavy objects. The mast (the tall vertical part of a crane) is supported by a large, heavy base, which helps offset the weight the crane holds at higher heights. At the top of the mast, there is a slewing unit, consisting of a gear and motor, which allows the crane to rotate. The slewing unit also contains the jib (a long horizontal working arm) that uses a trolley to help carry the load, as well as a shorter machinery arm that contains the crane's motor, along with large concrete counter weights to help balance the load. In addition to these simple machines, mobile cranes also utilize a pulley system, which helps the crane lift heavier loads with minimal force.