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Work Energy Kinetic Energy Potential Energy Mechanical Energy Conservation of Mechanical Energy

Work Energy Kinetic Energy Potential Energy Mechanical Energy Conservation of Mechanical Energy. ENERGY. The capacity to do work The energy transferred to an object equals the work done on the object  E T = W. Energy is present in many forms Heat (thermal energy)

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Work Energy Kinetic Energy Potential Energy Mechanical Energy Conservation of Mechanical Energy

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  1. Work • Energy • Kinetic Energy • Potential Energy • Mechanical Energy • Conservation of Mechanical Energy

  2. ENERGY • The capacity to do work • The energy transferred to an object equals the work done on the object  ET = W

  3. Energy is present in many forms • Heat (thermal energy) • Kinetic Energy = “Motion Energy” • Potential Energy = “Stored Energy” • Mechanical energy – Sum of Potential and Kinetic • Nuclear • Sound • Electromagnetic energy • Chemical energy

  4. The parallel component does work The (F cos ) component does work

  5. NO work done if there: • Is no movement • The force if perpendicular to the displacement

  6. In which photo(s) is WORKbeing done? no yes yes no

  7. If a 10 Newton force displaces a 20 kg block 40 meters calculate the work done on the block by the force. F = 10N EXAMPLE The Normal force (FN ) and weight (mg) do no work in this problem, WHY?

  8. Work = Force  Distance • The component of force parallel to the displacement DOES WORK • The perpendicular component DOES NO WORK •  - Between the force and displacement • Scalar • Measured in Joules (J)

  9. Positive Work - Force and displacement in same direction. Negative Work - Force opposite the displacement. Zero Work - Force is perpendicular to the displacement. F d F d F d

  10. If the Force is in the Direction of Motion: In this case,  = 0 o, so:                  If the Force is in the Opposite Direction of Motion: In this case,  = 180o, so:

  11. If the Force is Perpendicular to the Direction of Motion: In this case,  = 90 o, so:                 If the Object Being Pushed Doesn't Move: In this case, x = 0, so:

  12. F= 10 N 300 A 10 N force acts 300 above the horizontal and displaces an object 5 meters horizontally how much work is done? W= F d cos  = (10N)(5m) cos 300 = 43.3 J

  13. How much work does gravity do on a 70 kg person who falls 100 meters in the free fall ride?

  14. Kinetic energy (KE) is the energy of a moving object. • Energy associated with motion and mass.

  15. A 500 kg car is traveling at 10 m/s, 10 sec. later it is traveling 30 m/s. Calculate the following: • The initial kinetic energy • The final kinetic energy • The change in KE

  16. Work Energy Theorem: The change in kinetic energy equals the work done.

  17. The animation shows a block of mass and initial speed v sliding across a floor that is not frictionless. A kinetic friction force fk stops block during displacement d. Thus we can relate work done by friction to the change E in the system's energy

  18. For hammer: Moving hammer can do work on nail! For nail:

  19. Matt’s little red wagon with a mass of 4.6 kg moves in a straight line • on a frictionless horizontal surface. It has an initial speed of 10 m/s • and is pulled by Matt 4.0 m with a force of 18N in the direction of • the initial velocity. Use the work-energy relation • (WNet = KE) to calculate the wagons: • Change in Kinetic Energy • Final speed.

  20. In a test of old sports car, it’s found that engines provided around 1,000 N of force. If the typical mass is 400 kg and they accelerate from rest, how fast will they be going 100 m down the road?

  21. How much work is done holding a box in place on an incline? How much work is done pushing a 15 kg box up a 30° incline at a constant speed for 3 m

  22. Force vs. Distance Area under curve equals the work done

  23. Force vs. Distance Area under curve equals the work done

  24. Area above the curve – work is positive Area below the curve – work is negative

  25. Work done by a variable force equals Area under the curve

  26. Gravitational Potential Energy (PE): The energy an object has due to its height above a reference point. The potential energy change is independent of the path between the initial and final points. link

  27. PE=mgh Can gains Potential Energy equal to mgh

  28. One serving of Bagel Crisps contains 543 kJ. How many pull-ups are needed to burn it off? • M = 60 kg ∆h = .5 m • ∆PE = mg ∆H • 543000J = (60kg)(9.81N/kg)(.5m)(n) • 1845 = n • But the human body is only abour 20% efficient so, n is only 369!

  29. Who does more work in lifting the respective equal masses to the top of the incline at a constant speed?

  30. 1. His initial potential energy with respect to the ground 2. His potential energy 1 seconds after being released 3. His change in potential energy 4. Where did its energy go when it hits the ground? Rufus the 5 kg cat falls 10 meters from above the surface of the earth. Calculate:

  31. Conservative Force: The work done is independent of the path taken. Only depends on the initial and final position. Ex: Gravity Non Conservative Force: The work done depends on the path taken. Ex: Friction

  32. Don’t disregard the Conservation Laws

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