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Conservation of Energy

Conservation of Energy. Forms of energy. You add energy to a system by doing work on it. If you do work on an object, it may: Accelerate…adding kinetic energy Be lifted…adding gravitational potential energy Bend…adding elastic potential energy. Work = force x distance.

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Conservation of Energy

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  1. Conservation of Energy

  2. Forms of energy • You add energy to a system by doing work on it. • If you do work on an object, it may: • Accelerate…adding kinetic energy • Be lifted…adding gravitational potential energy • Bend…adding elastic potential energy • Work = force x distance • The work you do is the energy it gains!

  3. How much work is done? A car exerts a force of 2500 N against the road as it accelerates over 63 m. 2500 N 63 m

  4. How much work is done? A 1200 kg car accelerates at 4.1 m/s2 over a distance of 75 m. a=4.1m/s2 1200kg 75m

  5. How much work is done? A forklift raises a 250 kg crate 3.2 m above the ground. 3.2m 250kg

  6. How much work is done? A 75 kg sprinter reaches 11 m/s at 3.1 s in a race. 75kg vf=11 m/s at 3.1 s

  7. How much work is done? • A car exerts a force of 2500 N against the road as it accelerates over 60 m. • A 1200 kg car accelerates at 4.1 m/s2 over a distance of 75 m. • A 75 kg sprinter reaches 11 m/s at 31 m in a race. • A forklift raises a 250 kg crate 3.2 m above the ground.

  8. Shall we return to the braking distance problem? When a car is braking, the tires do work against the road in the direction opposite to its motion. The work done decreases kinetic energy of the car. When KE=0 J, the car is at rest

  9. Shall we return to the braking distance problem? • The road can offer a car exactly 1/3 of its weight in friction with the tires at full braking. Suppose the car has a mass of 1100 kg (weight = 10800 N) • How much friction does the road provide? b) If the car is moving at 25 m/s, how much kinetic energy does it have? c) How far does it brake before KE0.0 J?

  10. Shall we return to the braking distance problem? • The road can offer a car exactly 1/3 of its weight in friction with the tires at full braking. Suppose the car has a mass of 1100 kg (weight = 10800 N) • How much friction does the road provide? b) If the car is moving at 50 m/s, how much kinetic energy does it have? c) How far does it brake before KE0.0 J?

  11. Triple axel • http://www.youtube.com/watch?v=XzFGzsn6Skg • Slam dunk • http://www.youtube.com/watch?v=fVmZnvdzAC0

  12. 55kg kid, 2.0 m fall.Make a table of GPE, KE, and velocity

  13. 55kg kid, 2.0 m fall.Make a table of GPE, KE, and velocity • Ht. GPE KE v • 2.0m • 1.5m • 1.0m • .5m • 0m

  14. How much work is done? • An 84 kg science teacher runs up Pikes Peak. The race includes a vertical climb of 2410 m.

  15. How much work is done? An 84 kg science teacher runs up Pikes Peak. The race includes a vertical climb of 2410 m.

  16. A student runs…

  17. A student runs… …or walks if he is a SLACKER! • …up three flights of stairs. How much work is done?

  18. A student runs… …or walks if he is a SLACKER! • …up three flights of stairs. How much work is done? • I dunno. What is YOUR mass, and how high is the lunchroom?

  19. Find three ways to measure the height of the lunchroom (above the first floor)

  20. Power Lab • Measure the change in height from the first to fourth floors. • Run (or walk) up the stairs, measure and record the time required • Calculate: Work done and power for this trip

  21. Momentum=mass x velocity p=m x v • The units of momentum are kg m/s

  22. In a collision: Momentum is conserved! Momentumbefore=Momentumafter p initial=p final

  23. Collisions Before: After:

  24. Elastic Collisions Before: After:

  25. Elastic Collisions Before: After:

  26. Elastic Collisions Before: After:

  27. Elastic Collisions Before: After:

  28. Elastic Collisions Before: After:

  29. Elastic Collisions Before: After:

  30. Elastic Collisions Before: After:

  31. What is the momentum of….? • A pitched baseball? • A sprinter? • A car on Louisiana Ave.? • A car on I-25? • A semi on I-25? • An aircraft carrier at the dock?

  32. What is the momentum of….? • A pitched baseball? (.15kg, 45m/s) • A sprinter? (60 kg, 10 m/s) • A car on Louisiana Ave.? (1000 kg, 15 m/s) • A car on I-25? (1000 kg, 35 m/s) • A semi on I-25? (15000 kg, 35 m/s) • An aircraft carrier at the dock? (200000000 kg, 0 m/s)

  33. What is the kinetic energy of….? • A pitched baseball? (.15kg, 45m/s) • A sprinter? (60 kg, 10 m/s) • A car on Louisiana Ave.? (1000 kg, 15 m/s) • A car on I-25? (1000 kg, 35 m/s) • A semi on I-25? (15000 kg, 35 m/s) • An aircraft carrier at the dock? (200000000 kg, 0 m/s)

  34. A change in momentum: …if momentum is mass x velocity— …and the mass of an object can’t change, then it’s a change in velocity

  35. A change in momentum: …if momentum is mass x velocity— …and the mass of an object can’t change, then it’s a change in velocity =acceleration x time =Force/mass x time

  36. Did you notice? While work is force x distance, a change in momentum is force x time! m x Dv=m x (a x t)=m x (F/m x t)=F x t

  37. Did you notice? While work is force x distance, a change in momentum is force x time! m x Dv=m x (a x t)=m x (F/m x t)=F x t • This is called an impulse: • Dp=mDv=Ft

  38. Try it. • If a 10. kg object at rest is pushed with a force of 20. N (total) for 10. s…

  39. Try it. • If a 10. kg object at rest is pushed with a force of 20. N (total) for 10. s… • The impulse is F x t

  40. Try it. • If a 10. kg object at rest is pushed with a force of 20. N (total) for 10. s… • The impulse is F x t = 20. N x 10. s = 200 Ns = 200 kg m/s

  41. Try it. • If a 10. kg object at rest is pushed with a force of 20. N (total) for 10. s… • The impulse is F x t = 20. N x 10. s = 200 Ns = 200 kg m/s • Did you notice? • The acceleration is 2.0 m/s2 • The final velocity is 20 m/s • The final momentum is 200 kg m/s

  42. Consider a 15 kg object, accelerated by a 30 N force. • If the force is applied for 100 m… • ?a • ?t • ?vf • ?p • ?KE • ?W • If the force is applied for 10 s… • ?a • ?d • ?vf • ?p • ?KE • ?W

  43. Crash test videos • http://www.youtube.com/watch?v=sh1-ti8cCiw • Introduction • http://www.youtube.com/watch?v=FnH_gvhI9OI • Basic crash • http://www.youtube.com/watch?v=d7iYZPp2zYY • Seat belt and airbag • http://www.youtube.com/watch?v=fPF4fBGNK0U • Old vsnew cars

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