lever(redirected from third-class lever)
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arrangement of moving and stationary mechanical parts used to perform some useful work or to provide transportation. From a historical perspective, many of the first machines were the result of human efforts to improve war-making capabilities; the term engineer
..... Click the link for more information. consisting of a bar supported at some stationary point along its length and used to overcome resistance at a second point by application of force at a third point. The stationary point of a lever is known as its fulcrum. The term lever is also applied to a projecting piece that is moved to operate or adjust inner machinery, such as a lever moved to the right or left to switch electric current on or off or to adjust the size of the opening of a shutter in a camera.
Principle of the Lever
It has been found by experiment that two equal forces acting in opposite directions, i.e., clockwise and counterclockwise, and applied to a uniform lever at equal distances from the fulcrum counteract each other and establish a state of equilibriumequilibrium,
state of balance. When a body or a system is in equilibrium, there is no net tendency to change. In mechanics, equilibrium has to do with the forces acting on a body.
..... Click the link for more information. , or balance, in the lever. Experiment has also shown that two unequal forces when acting in opposite directions will bring about an equilibrium when the product of the magnitude of one force and its effort arm, or lever arm (the distance of its point of application from the fulcrum), is equal to the product of the magnitude of the other force and its effort arm. In physics the product of a force by its effort arm is called a momentmoment,
in physics and engineering, term designating the product of a quantity and a distance (or some power of the distance) to some point associated with that quantity.
..... Click the link for more information. of the force; the general conclusion known as the principle of moments states that equilibrium is established when the sum of the moments of the forces acting in a clockwise direction is equal to the sum of the moments of the forces acting in a counterclockwise direction. It is possible, as a result, to overcome a very large force at a short distance from the fulcrum with a very small force at a great distance from the fulcrum. Archimedes is supposed to have boasted, having the lever in mind, that given a place to stand he could move the world.
Classification and Application of Levers
In the use of a small force to overcome a large one the lever finds its many common applications. The lever is used for prying, as in the case of the crowbar, or for lifting. For example, the fulcrum is the point upon which a crowbar rests when used to lift or to pry loose some object; the effort is applied at the end farther from the fulcrum and is relatively small. The distance from the operator's hands to the fulcrum is known as the lever arm, or effort arm; the object being pried loose is the resisting force, or resistance; the object's distance from the fulcrum is the resistance arm. Levers in which the fulcrum is located between the effort and the resistance, as in the crowbar and the beam balance, are known as first-class levers. The fulcrum may also be located at one end of the lever, with the effort applied at the other end and the resistance in between; this type of lever, illustrated by the wheelbarrow and the nutcracker, is known as a second-class lever. The final possibility, known as a third-class lever, has the effort applied between the fulcrum and the resistance and is illustrated by various types of tongs.
Many other common tools, instruments, and appliances are applications of the principle of the lever. The human forearm is an application of the third-class lever, the elbow acting as the fulcrum, the weight held in the hand and being lifted as the resistance, and the pull of the muscles between the elbow and the hand as the effort. In a second-class lever, the effort arm is always longer than the resistance arm, so that a smaller effort moves a larger resistance, while in a third-class lever the reverse is always true, with the effort greater than the resistance. In a first-class lever, the effort may be either larger or smaller than the resistance, depending upon the location of the fulcrum.
a simple machine consisting of a rigid element that pivots about a fixed support (fulcrum) and enables a smaller force to balance a larger one. The operating principle behind any type of lever can be expressed by the equality Ph1 = Qh2, where P and Q are the applied forces, and h1 and h2 are the lever arms, that is the perpendicular distances from the fulcrum to the line of action of each force.
If the fulcrum is positioned between the points where the forces are applied, the machine is called a first-class lever (Figure 1, a). If both forces are applied on the same side of the fulcrum, the machine is a second-class lever (Figure 1, b). With first-class levers, the forces must take the same general direction; with second-class levers, the forces assume different directions. Archimedes developed the theory of equilibrium for levers balanced by weights, and the general condition for lever equilibrium was worked out by P. Varignon in 1687. Levers are often used as simple lifting devices.