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Chapter 7

Chapter 7. Energy. Energy. Universe is made up of matter and energy Energy is the mover of matter. Energy has several forms: Kinetic, Potential, Electrical, Chemical, etc. Work. Work = Force X Distance W = Fd Measured in units of Newton-meters or Joules. Work Questions .

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Chapter 7

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  1. Chapter 7 Energy

  2. Energy Universe is made up of matter and energy Energy is the mover of matter. Energy has several forms: Kinetic, Potential, Electrical, Chemical, etc.

  3. Work • Work = Force X Distance W = Fd • Measured in units of Newton-meters or Joules

  4. Work Questions • How much work is done when a weight lifter lifts a barbell weighing 1000 N to a height of 1.5 m? • How much work is done when a weight lifter pushes on a stationary wall with a force of 1000 N for 15 seconds?

  5. Power • Power = (work done) / time • Power is measured in units of joules/sec or Watts

  6. Light Bulbs and Power • How much energy does a 100 W light bulb use in one hour? • 100 W = 100 joules/sec = 360,000 joules/hour • so in one hour we use, • (360,000 joules/hour)X(1 hour) = 360,000 J • of energy

  7. Mechanical Energy • The ability to do work is called energy and it has the same units as work (Joules) • Lifting a weight from the floor requires work (Force * distance). It also gives the weight energy. (Release the weight and what happens?)

  8. Kinetic Energy • Energy of Motion • KE = ½ (mass) X (velocity)2 • KE = ½ mv2 • Question: How much energy does a 1 kg mass have if it is moving at 10 m/s?

  9. Kinetic Energy • Answer: • m = 1 kg v = 10 m/s • KE = ½ (1 kg) (10 m/s)2 • KE = 50 J

  10. Kinetic Energy Question • You have a choice of catching a baseball or a bowling ball, both with the same KE. Which is safer? KE = ½Mv2 KE = ½mV2 p2 = mV p1 = Mv

  11. Solution • ½Mv2 = ½mV2 , so But

  12. Work - Energy Theorem • If you want to move something, you are giving it kinetic energy and you must do work on it. • The work done is equal to the change in the kinetic energy W = DKE

  13. Potential Energy • Stored energy is called potential energy • Examples:Rubber band Springs Bow Batteries (chemical energy)

  14. Gravitational Potential Energy • PE = Weight X Height PE = mgh

  15. Gravitational Potential Energy Notes: Only the vertical height matters (you are doing work against gravity which only acts vertically).Independent of path Only the difference in potential energy matters. You are free to set PE = 0 at any point.Arbitrary zero point

  16. Total Mechanical Energy E = KE + PE

  17. Conservation of Mechanical Energy • Total mechanical energy cannot be changed so long as the system has no dissipative forces (friction, air resistance). • If the system is not losing energy to heat, then mechanical energy is conserved.

  18. Conservative Forces Gravity Springs

  19. Simple Machines • Devices that amplify forces • If there are no losses, then Work in = Work out (FD)input = (FD)output

  20. F f Simple Machines • Levers d D • Inclined Planes

  21. Simple Machines • Pulleys f

  22. Conservation of Energy • Throw a ball into the air with an initial velocity of 10 m/s. • How high does it get?

  23. How High does it Get? KE = 0 PE = mgh V = 0 E = mgh ½mv2 = mgh h = v2/2g = (10 m/s)2/2*10m/s2 = 5 m KE = ½mv2 PE = 0 E = ½mv2 V = 10 m/s

  24. Efficiency • Efficiency = work done/energy used Useful energy becomes wasted energy when it is spent in heat. Heat is the graveyard of useful energy.

  25. Example Questions - Chapter 7 A 10 lb weight is lifted 5 ft. A 20 lb weight is lifted 2.5 ft. Which lifting required the most work? (a) 10 lb weight (b) 20 lb weight (c) same work for each lifting (d) not enough information is given to work the problem (c) same work for each lifting

  26. An object of mass 6 kg is traveling at a velocity of 30 m/s. How much total work was required to obtain this velocity starting from a position of rest? a) 180 Joules b) 2700 Joules c) 36 Joules d) 5 Joules e) 180 N W = DK W = ½mv2 – 0 W = ½ (6 kg) (30 m/s)2 W = 3*900 = 2700 Joules b) 2700 Joules

  27. Two cars, A and B, travel as fast as they can to the top of a hill. If their masses are equal and they start at the same time, which one does more work if A gets to the top first? • A • B • they do the same amount of work

  28. A 20 pound weight is lifted 4 feet. The change in potential energy of the weight in ft.lb is • 20 • 24 • 16 • 80 • 5

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