Levers
Suppose you need to pry the lid off a can of paint. How can you make the task easier? A common solution is to slip the flat end of a screwdriver under the lid of the paint can and then to pry the lid off by pushing down on the screwdriver. This is an example of a lever, a rigid bar that is free to move around a fixed point. The fixed point the bar rotates around is the fulcrum. Levers are classified into three categories based on the locations of the input force, the output force, and the fulcrum.
The input arm of a lever is the distance between the input force and the fulcrum. The output arm is the distance between the output force and the fulcrum. 
To calculate the ideal mechanical advantage of any lever, divide the input arm by the output arm.
First-Class Levers Figure 13A shows a screwdriver being used as a first-class lever to open a paint can. The fulcrum in this case is actually the inside edge of the paint can. The position of the fulcrum identifies a first-class lever—the fulcrum of a first-class lever is always located between the input force and the output force.
Depending on the location of the fulcrum, the mechanical advantage of a first-class lever can be greater than 1, equal to 1, or less than 1. Examples of first-class levers include a seesaw, scissors, and tongs.
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