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Energy

Energy. conservation of energy work, energy, and power machines & efficiency Homework: RQ: 3, 4, 5,10, 12, 13, 15, 18, 30. Ex: 23, 26, 28, 37, 49, 62. Problems: 1, 5, 6. Energy & Work. Energy is the capacity to do ___________ Unit: joule = newton·meter (J = N·m)

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Energy

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  1. Energy • conservation of energy • work, energy, and power • machines & efficiency • Homework: • RQ: 3, 4, 5,10, 12, 13, 15, 18, 30. • Ex: 23, 26, 28, 37, 49, 62. • Problems: 1, 5, 6.

  2. Energy & Work • Energy is the capacity to do ___________ • Unit: joule = newton·meter (J = N·m) • Work = force x distance (Fd) when force is in direction of motion (or opposite to motion) • Ex. 50N pushes distance of 4 meters. • W = (50N)(4m) = _______________ • /

  3. Machines • change an applied force by ___________ it, _________________________ it, or ______________________ its direction. • Types: • inclined plane, screw, wedge • pulley, wheel • lever

  4. levers • Work input Fd = Work output Fd • Ex. Your hand moves 100m, causes car to rise 0.10m. The force amplification factor is, F d __ __ = F d

  5. inclined plane • Weight x height change • = Force x distance along plane • Force along ramp _________ than Weight • Ramp distance ____________ than height change • ADA Standards: Ramp must be at least 12x longer than vertical rise • Ex. A 1ft vertical rise requires ______ of ramp.

  6. Machine Efficiency • = (work output)/(energy input) x 100% • Ex: 10J are input and 9J are output. • Efficiency = (9)/(10) x 100% = _______ • Ex: 5J are input and 4J are output. • Efficiency = (4)/(5) x 100% = ________ • /

  7. Energy of Motion • Called Kinetic Energy (KE) • KE = ½(mass)(velocity)2 = ½mv2. • Ex. 2000kg car moving at 2m/s. • KE = ½ (2000)(2)2 = ___________

  8. Work & Energy • Work transforms energy from one ______ to another • Work = DKE • Ex. Calculate distance 100N must act to move a 2000kg car from rest to 2m/s: • Work = Fd = (100N)(d) = 4000 J • d = 4000J/100N = _____________ • //

  9. Power • Power is the ________ work is performed • Power = work/time = _________________ • Unit: watt = joule/second = J/s • Other Unit: horsepower • 1 horsepower = 746 watts • /

  10. Energy & Power • Energy = power x time • Ex. A toy car has 1000 J of energy at full charge. • How long can it run at 100 watts? At 10 watts? • Time = Energy/power • = 1000J/100watts = 10 seconds • = 1000J/10watts = 100 seconds/

  11. Energy & Efficiency • 1 gallon gasoline has ________________ • Engines only get a fraction of this: • Ex. A 25% efficient car gets (0.25)(138,000,000 J) = 34,500,000J out of 1 gallon. • A 20% efficient car gets 27,600,000J.

  12. Mpg (20% Efficient Engine) • Work = Force x distance • Ex. 400N for 1600 meters (1 mile) • Work = (400N)(1600m) = 640,000J for one mile traveled (1mile/640,000J) • Engine gets 27,600,000J per gallon • (at constant speed)

  13. Stop & Go Mpg • Energy is used to speed car, and all is _________ to _________ when stopping • Mpg much less in stop & go conditions • /

  14. Size, Shape & Mpg • Block shape creates __________ air friction than rounded shape car • Larger vehicles experience ________ air friction • Air friction = Shape factor x Frontal Size • Ex. At 60mph, an SUV can experience about 4x more air drag than a small car. • /

  15. Speed & Mpg • For a given vehicle, air friction increases with the ____________ • Ex. If you double your speed, the air friction will increase by a factor of 4. • /

  16. Potential Energy • … is energy due to __________________ • Ex. Book standing on one end has more potential energy than when lying flat • Ex. A ball 1m above floor has more potential energy than when on the floor.

  17. Gravitational Potential Energy • = mass x gravity x height • Ex. A 2kg ball is 1m above the floor • Grav. Pot. Energy = (2kg)(10N/kg)(1m) • = 20 joules • Ex. A 10kg sack of rice 0.5m above the floor has Grav. Pot. Energy • = (10kg)(10N/kg)(0.5m) • = 50 joules

  18. Conservation of Energy • Energy cannot be created or destroyed; but is __________________ from one form into another – the total amount staying the same. • Ex. A falling object _____________ Gravitational Energy as it falls, but ___________ an equal amount of Kinetic Energy. • /

  19. Mechanical Energy • = sum of ___________________ Energy • Ex. A glider slides down an inclined air track. The Mech. Energy = KE + mgh = constant as the glider moves to lower heights h. • /

  20. Summary • work = Fd (F along d) • work = 0 (F perpendicular to d) • Power = work/time = Fv • KE = ½mv2. GPE = mgh • work = change in KE • total energy always conserved • machines & efficiency

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