simple machineTeaching Simple Machines

Energy conservation.Ignoring for a moment the losses of energy due to friction, the work done on a simple machine is the same as the work done by the machine to perform some sort of task. If work in equals work out, then the machine is efficient.

Machines simply transmit mechanical work from one part of a device to another part. A machine produces force and controls the direction and the motion of force, but it cannot create energy. A machines ability to do work is measured by two ctors. These are mechanical advantage and efficiency.

The ideal mechanical advantage IMA of a pulley is directly dependent upon the number of support strings, N.

Pulley.A pulley is a wheel over which a rope or belt is passed. It is also a form of the wheel and axle. Pulleys are often interconnected in order to obtain considerable mechanical advantage.

combine two or more levers, usually to decrease the effort. By applying the principle of the compound lever, a person could use the weight of one hand to balance a load weighing a ton.

The ideal mechanical advantage IMA of an inclined plane is the length of the incline ided by the vertical rise, the socalled runtorise ratio. The mechanical advantage increases as the slope of the incline decreases, but then the load will have to be moved a greater distance. Again, work in equals work out in an entirely efficient system. Friction will be large if objects are slide along the suce of the inclined plane. Efficiency can be increase by using rollers in conjunction with the inclined plane.

FE. If the two arms of the lever are of equal length, the effort must be equal to the load. To lift pounds, an effort of pounds must be used. If the effort arm is longer than the load arm, as with a crowbar, the hand applying the effort travels rther and the effort is less than the load. SOCIAL CONTEXT Seesaws, crowbars, and equalarm balances are examples of a first class lever; a pair of scissors is a double lever of the first class.

. The hand applying the effort always travels a shorter distance and must be greater than the load. SOCIAL CONTEXT The forearm is a thirdclass lever. The hand holding the weight is lifted by the bicep muscles of the upper arm that is attached to the forearm near the elbow. The elbow joint is the fulcrum.

Screw.The screw is actually an inclined plane wrapped in a spiral around a shaft. A jackscrew combines the usefulness of the screw and the lever. The lever is used to turn the screw.

A machine is a device that does work. Most machines consist of a number of elements, such as gears and ball bearings, that work together in a complex way. Nonetheless, no matter how complex they are, all machines are based in some way on six s of simple machines. These six s of machines are the lever, the wheel and axle, the pulley, the inclined plane, the wedge, and the screw.

Lever.A lever is a bar resting on a pivot. Force effort applied at one point is transmitted across the pivot fulcrum to another point which moves an object load.

Inclined plane.The inclined plane is a simple device that hardly looks like a machine at all. The mechanical advantage increases as the slope of the incline decreases. But the load will then have to be moved a greater distance.

Wedge.The wedge is an adaptation of the inclined plane. It can be used to raise a heavy load over a short distance or to split a log.

Secondclass levers have the load located between the fulcrum and the effort F

A lever is in equilibrium when the effort and the load balance each other; that is, the sum of the torques force times lever arm equals zero. The effort multiplied by the length of the effort arm equals the load multiplied by the length of the load arm.

Firstclass levers have the fulcrum located between the load and the effort

Wheel and axle.The wheel and axle is essentially a modified lever, but it can move a load rther than a lever can. The center of the axle serves as a fulcrum.

Thirdclass levers have the effort located between the load and the fulcrum FE

The ideal mechanical advantage IMA of a screw is ideally the ratio of the circumference of the screw to the distance it advances during each revolution. Machine screws, working their way through a nut, can be relatively efficient. Wood screws tend to be anything but efficient as considerable amounts of energy are lost to friction and the displacement of matter. A jackscrew, such as those used to raise homes and other structures, combines the usefulness of the screw and the lever. The lever is used to turn the screw. The mechanical advantage of a jackscrew is quite high.

Resource Information for Teaching Simple Machines

The ideal mechanical advantage IMA ignoring internal friction of a lever depends on the ratio of the length of the lever arm where the force is applied ided by the length of the lever are that lifts the load. The IMA of a lever can be less than or greater than depending on the class of the lever. There are three classes of levers, depending on the relative positions of the effort is applied, load, and fulcrum.

Mechanical advantage.In machines that transmit only mechanical energy, the ratio of the force exerted by the machine to the force applied to the machine is known as mechanical advantage. Under mechanical advantage the distance the load will be moved will be only be a fraction of the distance through which the effort is applied. While machines can provide a mechanical advantage of greater than . and even less than . if desired, no machine can never do more mechanical work than the mechanical work put into it.

Efficiency.The efficiency of a machine is the ratio between the work it supplies and the work put into it. Although friction can be decreased by oiling any sliding or rotating parts, all machines produce some friction. A lever has a high efficiency due to the ct that it has low internal resistance. The work it puts out is almost equal to the work it receives, because energy used up by friction is quite small. On the other hand, an a pulley might be relatively inefficient due to a considerably greater amount of internal friction. Simple machines always have efficiencies of less than . due to internal friction.

The ideal mechanical advantage IMA of a wedge depends on the angle of the thin end. The smaller the angle, the less the force required to move the wedge a given distance through, say, a log. At the same time, the amount of splitting is decreased with smaller angles.

The ideal mechanical advantage IMA of a wheel and axle is the ratio of the radii. If the effort is applied to the large radius, the mechanical advantage is R/r which will be more than one; if the effort is applied to the small radius, the mechanical advantage is still R/r, but it will be less than .

E. As in a wheel barrow, the axle of the wheel is the fulcrum, the handles represent the position where the effort is applied, and the load is placed between the hands and the axle. The hands applying the effort travel a greater distance and is less than the load. SOCIAL CONTEXT In addition to a wheelbarrow, a pry bar represents a secondclass lever. A nutcracker is a double lever of this class.simple machineTeaching Simple Machines

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