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# Per8_over - Preview of Period 8 Work Efficiency and Complex...

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Preview of Period 8: Work, Efficiency, and Complex Machines 8.1 Work and Efficiency of Machines How efficient are block and tackle systems, winches, and wheels and axles? 8.3 More Simple Machines How do hydraulic systems and gears work? 8.4 Complex Machines How are simple machines combined into complex machines? How are the efficiency and mechanical advantage of complex machines defined? 8-1

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Efficiency of Machines in in out out in out D F D F W W Efficiency = = W in = work put into the machine (joules or foot-pounds) W out = work done by the machine (joules or foot-pounds) Machines do not reduce the amount of work needed. Since some energy is wasted overcoming frictional forces, the amount of work required using a machine is greater than the amount of work required without a machine. (Example) A machine requires 2,000 joules of energy to raise a 20 kilogram block a distance of 6.0 meters. What is the machine’s efficiency? First, find the work done to raise the block: W = M g h = 20 kg x 9.8 m/s 2 x 6.0 m = 1,176 J Next, use the efficiency equation. The work in equals 2,000 joules. in out Work Work Eff = = 1,176 joules = 0.59 = 59% 2,000 joules 8-2
Wheels and Axles When wheels of different diameters are attached to an axle, they trade force for distance. With each turn of the axle both wheels rotate one turn. If the wheels have different diameters, the edge of each wheel turns a different distance and speed. The force that the rim of the small wheel can exert is greater than the force applied to the rim of the large wheel.

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Per8_over - Preview of Period 8 Work Efficiency and Complex...

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