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Energy

Learn about energy conservation, work, power, and efficiency in machines. Solve problems and explore the concepts of energy transformation, Kinetic Energy, Gravitational Potential Energy, and the conservation of mechanical energy.

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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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