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Energy, Work, and Simple Machines Chapter 10

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Energy = ability to produce a change in itself or its surroundings. Work transfer energy by mechanical means. Kinetic energy = energy of motion Ke = ½ mv 2 Work-Energy Theorem K = W

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Work Product of force and distance; scalar quantity. W = Fd Unit is J named after James Prescott Joule. 1 J = 1 N*m Work is done only if object moves in the direction of the applied force. Ex Prob 226 Prac Pr 227

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Work and Direction of Force Figure 10-4 p 227 shows work can be broken into its components and that work is done only in the component in which the force is applied.

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W = Fd cos Since cos 90 = 0, no work is done in the vertical direction. Work done by the friction of grass is negative. Positive work done by you and means you are working.

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Ex Prob 228 Pr Prob 229 On a Force vs Displacement Graph, the area under the curve is work. Fig Power = rate of doing work P = W/t

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Measured in watts (W) 1W = 1 J/s Often measured in kW because 1 W is so small. Ex Prob 230 Prac Pr 231

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10.2 Machines Eases load by changing magnitude or direction of force, but it does not change the amount of work done. Figure Example of Simple MachinesFigure

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Simple and Complex Machines Work you do = Wi Work machine does = Wo

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Effort force, Fe - force you exert on machine Resistance force, Fr - force exerted by machine Mechanical Adv = Fr / Fe A machine can increase force, but not energy.

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Simple Machines Pulleys & Pulley SystemPulleysSystem Inclined PlaneInclined Plane Block & Tackle Levers –1 st Class, 2 nd Class, 3 rd Class1 st 2 nd Class3 rd

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Ideal machine transfers all energy Wo = Wi. Fr dr = Fe de Fr / Fe = de / dr MA = Fr / Fe IMA = de /dr Efficiency = Wo / Wi x 100% = MA / IMA x 100%

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Lower efficiency - greater effort is needed to exert the same Fr. IMA = ratio of distances moved.

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Compound Machines Consists of two or more simple machines. Resistance force of one becomes the effort force of the second. MA of complex = product of MA of each in compound

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Ex Prob 237 Pr Prob 238 Movement by body is explained by principles of force and work. Lever systems four parts. Fig

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1. Rigid bar (bone) 2. Source of force (muscle contraction) 3. Fulcrum (movable joints between bones) 4. Resistance (weight of body or object being moved)

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Tall person has lever with lower MA than short person. They must apply a greater force to move the longer lever formed by the leg bones. Tall people rarely have stamina in walking races.

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Fig Simple Machines

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