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There are three classes of levers, all used for different purposes. Every lever class has a fulcrum, which is what a lever rests on. A lever is simply a bar placed on a fulcrum or attached to a fulcrum.
Class One Levers
A see-saw is an example of a class one lever. This is because it has a fulcrum in the middle of the bar. A class one lever always has a fulcrum that it not on either end of the bar. A force applied on one side of the lever is exerted across the fulcrum on the other side of the bar. The input force on one end of the lever is magnified by the difference in the length of the lever from the fulcrum to the input and the length from the fulcrum to the output of the force. This means that small forces in the input can be magnified if the input is far from the fulcrum while the output is close to the fulcrum.
Class Two Levers
Class two levers must have the fulcrum at the very end of the lever bar. The fulcrum provides resistance to the input force. This means that the output force is in the same direction as the input and is between the input and fulcrum. An example of a class two lever is doing a push-up. The fulcrum is your feet. the input force is your hands pushing upward, and the output force is the upward motion of your body between your hands and your feet.
Class Three Levers
Class three levers are similar to class two levers, except that the fulcrum must be attached to the lever bar. This is because the output and input forces are switched in a class three lever, and the input force must exceed the output force. A shovel is probably the most common class three lever. The fulcrum would be your hand on the back of the shovel handle. The input would be the hand in the middle of the shovel handle lifting up, and the output would be the force acting on the material you are lifting.
A pulley is a mechanism in which a line, usually a rope, string or cable, passes through a low-friction wheel. Pulleys allow force to be redirected along the line of whatever passes through it. Pulleys perform work according to the following equation: work = force x distance. By increasing the distance, less force can be applied to do the same amount of work in moving an object. By placing many pulleys, all redirecting the force along a line, work can be done on very large objects using little force.
Pulleys are often used to create mechanical advantages, which are measures of how much a machine lessens the force needed to move a load. A mechanical advantage is created when a load can be pulled a longer distance with less force. This means that the force used to pull a line can move the line a short distance but move the load a much longer distance. This is achieved through the use of movable pulleys. As force pulls on one end, the pulleys move in order to reduce the distance that must be pulled to move the load an equivalent distance. This allows far less force to be used to accomplish much more work; however, it puts much more strain in the line.
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