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Energy, Work, and Simple Machines

Energy, Work, and Simple Machines. Chapter 10. 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. Work.

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Energy, Work, and Simple Machines

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  1. Energy, Work, and Simple Machines Chapter 10

  2. Energy = ability to produce a change in itself or its surroundings. • Work transfer energy by mechanical means. • Kinetic energy = energy of motion • Ke = ½ mv2 • Work-Energy Theorem K = W

  3. 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

  4. 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.

  5. 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.

  6. Ex Prob 228 Pr Prob 229 • On a Force vs Displacement Graph, the area under the curve is work. Fig 10-5 229 • Power = rate of doing work • P = W/t

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

  8. 10.2 Machines • Eases load by changing magnitude or direction of force, but it does not change the amount of work done. • Figure 10-9 235 Example of Simple Machines

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

  10. 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.

  11. Simple Machines • Pulleys & Pulley System • Inclined Plane • Block & Tackle • Levers • 1stClass, 2nd Class, 3rdClass

  12. 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%

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

  14. 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

  15. Ex Prob 237 Pr Prob 238 • Movement by body is explained by principles of force and work. • Lever systems four parts. • Fig 10-12 238

  16. 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)

  17. 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.

  18. Fig 10-9 235 Simple Machines

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