The fulcrum is the pivot point of the beam. When one end of the lever exerts effort, the opposite end of the lever experiences a load. The load creates a need for another point of contact, which is provided by the fulcrum. Fulcrums can be either fixed or movable. In a fixed fulcrum system, both the lever and the object it is manipulating remain in place. In a movable fulcrum system, something else must be manipulated to move the object being lifted or lowered. For example, with a block and tackle system, a person would be required to move the block to raise or lower an object - such as a tree - that is tied to it. Fixed fulcrums are useful when you want to manipulate something that cannot be moved, such as the mechanism of a machine. For example, the wheels on a cart cannot be rotated to change the alignment of the cart with respect to the direction it is being pulled. This would require a person to physically rotate the whole cart if a fixed-fulcrum lifting device were used instead. However, people have needs for mobile fulcrums as well, so fixed-fulcrum systems should not be viewed as completely obsolete.