2. Chapter 5 Energy

3. Energy Universe is made up of matter and energy. Energy is the mover of matter. Energy has several forms: –Kinetic –Potential –Electrical –Heat –etc.

4. Work a force for how long in distance. Work = Force  Distance W = F d The unit for work is the Newton-meter which is also called a Joule.

5. Work = Force || x Distance In this case, the distance is the magnitude of the displacement. Only the component of force parallel or anitparallel to the displacement does work

6. x

7. M= 10kg Mg = 100N In this case, the weight does positive work d = 2 m Work = Mgd = (100N)(2m) Work = 200 Nm =200J M= 10kg

8. Mg = 100N In this case, the weight does negative work d = 2 m Work = -Mgd = -(100N)(2m) Work = -200 Nm = -200J FaFa M= 10kg Mg = 100N FaFa

9. 1.WORK Now instead of a force for how long in time we consider a force for how long in distance. Work = Force x distance or W = F. d Units - Joules (J) or ft. lb BTU = 778 ft. lb (energy of one wooden kitchen match) Pushing on a wall and wall doesn’t move (no work done on the wall)

10. Questions: How much work is done when a weight lifter lifts a barbell weighing 1000 Newtons 1.5 meters above the ground? How much work is done when a weight lifter pushes on a stationary wall with a force of 1000 Newtons for 15 seconds?

11. Power Power is equal to the amount of work done per unit time. The unit for power is the Joule/second which is also called a Watt.

12. POWER Power = Work/time or P = W/t Units - J/s = W 550 ft. lb /s = 1 hp 1 hp = 750 J/s = 750 W 1 BTU/hr =.293 W 100 W bulb =0.1333 hp 250 hp engine = 187,500 W

13. Light Bulbs and Appliances How much energy does a 100 Watt light bulb use in one hour? How about a 40 Watt light bulb? Which bulbs shines brighter?

14. Mechanical Energy When work is done on an object, the object generally has acquired the ability to do work. This "ability to do work" is called energy and it has the same units as work….Joules. Two Types of Mechanical Energy –Potential Energy and Kinetic Energy

15. Potential Energy The energy that is stored is called potential energy. An object’s ability to do work by virtue of its position. Examples: –Rubber bands –Springs –Bows –Batteries –Gravity?

16. Gravitational Potential Energy PE = Weight  height PE = m g h Question: –How much potential energy does a 10kg mass have relative to the ground if it is 5 meter above the ground?

17. mg h h How much work does gravity do on the falling mass? Work = mgh PE = mgh

18. mg h h How much energy does the mass have at the bottom just before it hits the ground? Work = mgh PE = mgh v

19. Kinetic Energy Kinetic Energy is the energy of motion. Kinetic Energy = ½ mass  speed 2 Question: How much kinetic energy does a 1kg mass have if it is moving at 10 meters/second?

20. mg h How much energy does the mass have at the bottom just before it hits the ground? Work = mgh v mg This is the kinetic energy KE = ½ mv 2 PE = mgh

21. Energy is conserved

22. Kinetic Energy Depends on Speed: In fact, your kinetic energy is proportional to the square of your velocity - if you go twice as fast, your kinetic energy quadruples. If you go three times as fast,you have nine times the kinetic energy. Kinetic Energy Depends on Mass: Your kinetic energy is proportional to your mass. If mass doubles, kinetic energy doubles. If mass triples, kinetic energy triples, too.

23. Difference between momentum and Kinetic energy

24. Scalar Versus Vector: An important difference is that momentum is a vector quantity - it has a direction in space, and momenta combine like forces do. Kinetic energy is a scalar quantity - it has no direction in space, and kinetic energies combine like "regular numbers

25. Dependence on Velocity: The momentum of an object is proportional to the object's velocity - if you double its velocity, you double its momentum. The kinetic energy of an object is proportional to the square of the object's velocity - if you double its velocity, you quadruple its velocity.

26. A Thought Experiment: Suppose that you were captured by an evil physicist who gave you the following choice: You must either: Stand in front of a 1000 kg. truck moving at 1 m/s, or Stand in front of a 1 kg. frozen meatball moving at 1000 m/s.

27. Truck: Truck's momentum = mv = (1000 kg)(1 m/s) = 1000 kg m/s Truck's kinetic energy = 0.5 mv2 = (0.5)(1000 kg)(1 m/s)2 = 500 Joules Meatball: Meatball's momentum = mv = (1 kg)(1000 m/s) = 1000 kg m/s Meatball's kinetic energy = 0.5 mv2 = (0.5)(1 kg)(1000 m/s)2 = 500 000 Joules

28. PE = 1000J KE = 0J PE = 800J KE = 600J KE = 1000J KE = 200J PE = 400J PE = 0J

29. PE = 500J KE = 0J PE = 100J KE = 400J PE = 0J KE = 500J PE = 100J KE = 400J

30. Machines Fulcrum Mg Work = F D = N d D d M N F

31. Work/Energy Relationship If you want to move something, you have to do work. The work done is equal to the change in kinetic energy. Work =  KE

32. Example Question When the brakes of a car going 90 km/h are locked, how much farther will it skid than if the brakes lock at 30 km/h? Answer: 9 times

33. 4.CONSERVATION OF ENERGY Galileo's inclines Demo - Loop the loop Demo - Loop the loop Energy lost due to friction is actually not a loss; it is just a conversion.

34. Conservation of Energy Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes. Demos –Galileo's incline –Bowling ball pendulum –Loop the loop

35. Example Problem A 100 kg mass is dropped from rest from a height of 1 meter. How much potential energy does it have when it is released? How much kinetic energy does it have just before it hits the ground? What is its speed just before impact? How much work could it do if it were to strike a nail before hitting the ground?

36. 100 kg 1 meter nail

37. Machines A device used to multiply forces or simply to change the direction of forces

38. Components of Machines Lever …a bar that is free to pivot, or turn, about a fixed point Fulcrum….Fixed point of the Lever Effort Arm….Part of lever on which the effort force is applied Resistance arm…. Part of the lever that exerts the resistance force

39. Type I or First Class Lever The fulcrum between the force and the load, or between input and output.

40. Type II or Second Class Lever Reverse the position of the load and the fulcrum. The load is in between the fulcrum and the effort force.

41. Type III or Third Class Lever Fulcrum is at one end the load is at the other end….Input force is applied between them

42. Pulley A Kind of a lever that can be used to change the direction of a force.

43. Mechanical advantage The ratio of resistance force to effort force MA = F r /F e F e ….effort force F r …..resistance force

44. Ideal Mechanical advantage Workout = Workin F r d r = F e d e IMA = d e /d r

45. Efficiency Workout = Workin efficiency = w o / W i X 100 %

46. Compound Machines Consists of two or more simple machines linked….. so that the resistance force of one machine……… becomes the effort force of the second.

47. Machines - An Application of Energy Conservation If there is no mechanical energy losses then for a simple machine... work input = work output ( F d )input = ( F d )output Examples - levers and tire jacks

48. Efficiency Useful energy becomes wasted energy with inefficiency. Heat is the graveyard of useful energy.

49. Comparison of Kinetic Energy and Momentum Kinetic energy is a scalar quantity. Momentum is a vector quantity. Discuss rubber bullets as compared to lead bullets.

50. Example Questions 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

51. 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 (b) 2700 Joules

52. A 20 Newton weight is lifted 4 meters. The change in potential energy of the weight in Newton. meters is (a) 20 (b) 24 (c) 16 (d) 80 (e) 5 (d) 80