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5-2 Notes

5-2 Notes. Energy. Kinetic Energy. Kinetic energy is the energy due to motion. KE = ½ mv 2 Units for energy is the joule (J). Work-kinetic energy theorem. The net work done on an object is equal to the change in kinetic energy of object. W net = ½ mv 2 f – ½ mv 2 i

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5-2 Notes

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  1. 5-2 Notes Energy

  2. Kinetic Energy • Kinetic energy is the energy due to motion. • KE = ½ mv2 • Units for energy is the joule (J).

  3. Work-kinetic energy theorem • The net work done on an object is equal to the change in kinetic energy of object. • Wnet = ½ mv2f – ½ mv2i • F x d= ½ mv2f – ½ mv2i

  4. Potential Energy • Potential energy is stored energy. • Gravitational Potential energy is the energy due to the position of the object. • PE = mgh • Units are joules (J)

  5. Elastic Potential Energy • Elastic potential energy in a stretched or compressed elastic object. • PEelastic = ½ kx2 • PE = Joules (J)

  6. Spring Constant • k = spring constant • Spring constant expresses how resistant a spring is to being compressed or stretched. • Flexible spring = small k • Stiff Spring = large k • Spring constant = N/m

  7. Practice 5B • #1. Calculate the speed of an 8000 kg airliner with KE of 1.1 x 109 J. • Knowns? • m = 8000 kg, KE = 1.1 x 109 J • Unknown? • V = ? • Equation? • KE = ½ mv2 • 1.1 x 109 J = .5(80000)v2 = 117 m/s

  8. Sample Problem 5C • A person kicks a 10kg sled, giving an initial speed of 2.2m/s. How far does it move if the μk = .1? • Known? • m = 10kg, vi = 2.2m/s, vf = 0, μk = .1 • Unknown? • d = ? • Equation? F x d= ½ mv2f – ½ mv2i • F = Ff since a is constant; Ff = μk·Fn; Fn = mg • F = .1(10)9.8 = 9.8; work it out and get 2.4 m

  9. Sample Problem 5D • A 70 kg man is attached to a bungee cord with an unstretched length of 15m. He jumps off a bridge that is 50 m high. When jumping, his cord stretches to 44m. The spring constant of the cord is 71.8 N/m. What is the total potential energy relative to the water? • Knowns? • m = 70, k = 71.8, g = 9.81, h = 50 – 44 = 6, x = 44 – 15 = 29 • Unkown? • Gravitational PE and Elastic PE • Equations? PE = mgh and PEelastic = ½ kx2 • PE = 70(9.8)(6) = 4100 • PE elastic = .5(71.8)(292) = 30200 • Total PE = 4100 + 30200 = 34300 J

  10. 5-3 Notes Conservation of Energy

  11. When we say something is conserved, we mean it remains constant. • If we have a certain amount of a conserved quantity, we will have the same amount at a later time. • This does not mean the quantity can’t change forms though.

  12. Mechanical Energy • The description of motion of objects involves the combination of kinetic energy and potential energy. • Mechanical energy is the sum of kinetic energy and potential energy. • ME = KE + PE

  13. Energy Classification

  14. Conservation of ME • MEi = MEf • ½ mv2i + mghi = ½ mv2f + mghf • Where friction is involved, the principal of conservation of mechanical energy is not true because kinetic energy is not all converted potential energy but some is lost to heat and sound.

  15. Practice 5E • #1. A bird is flying with a speed of 18 m/s over water and drops a 2 kg fish. If the bird is 5.4 m high, what is the speed of the fish when it hits the water? • Known? • Vi = 18m/s, m = 2kg, hi = 5.4m hf = 0 • Unknown? • Vf = ? • Equation? • ½ mv2i + mghi = ½ mv2f + mghf • .5(2)(182) +2(9.8)(5.4) = .5(2)vf2 +2(9.8)0 = 20.7m/s

  16. Assignment • 5.2 and 5.3 Worksheets

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