A lever may be nothing more than a rigid bar of some material (like wood or metal) that is supported at a single point (called the fulcrum), but the tool itself is far more intriguing than it may at first sound. Levers can be combined with other simple machines like wheels to make complex machines like wheelbarrows, but a lever by itself can be used for an amazing variety of tools, toys and devices.
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In a first-class lever---generally considered the simplest simple lever---the fulcrum is located somewhere between the two ends of the bar; force is exerted at one end and the resistance (or weight) which the lever overcomes is placed at the other end. First-class levers are different from other types because they reverse the direction of force---for example, you push down to raise something up---and if you move the fulcrum closer to the resistance you want to move, you can apply more force with less effort. We often think of these levers as prying tools---crowbars and can openers are common first-class levers. See-saws are often the first lever we encounter during childhood. Even the lift tab on a drink can is a first-class lever.
With a second-class lever, the fulcrum is located at one end of the bar, and the force is exerted at the other end; the resistance (or weight) is placed in-between them. With these levers, the direction of force is the same as the applied force; when you push down on the lever, you apply a downward force. These levers are often used for lifting or crushing things---bottle openers lift the lid from the bottle's neck, while nutcrackers crush shells. Wrenches also fall into this category, with the bolt to be loosened or tightened (the fulcrum) at one end, your hand (the force) at the other end, and the resistance between them being provided by the wrench itself as it connects the two.
Third-class levers often display the greatest variety of forms. The fulcrum is located at one end of the bar, and the resistance (or weight) is placed at the other end; the force is exerted in-between them. Levers are often used for hitting--baseball bats and golf clubs are common third-class levers used in sports, while hammers and axes are common tools. Third-class levers can also be used for digging or scooping; they can be as large as a shovel or as small as a fork or spoon. And perhaps the most recognisable third-class lever is your own forearm, with your bicep muscle (force) attached between your elbow (fulcrum) and wrist (weight).
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