LIVE INTERACTIVE LEARNING @ YOUR DESKTOP

1. LIVE INTERACTIVE LEARNING @ YOUR DESKTOP NSTA Press: Stop Faking It! Energy Work, Energy, and Simple Machines Dr. Bill Robertson Tuesday, May 5, 2009

2. NSTA Web SeminarEnergy: Stop Faking It! Bill RobertsonMay 5, 2009

3. Work, Energy, and Simple Machines

9. Which of the following describe what happens with the ruler? 9

10. Let’s Pause Two Minutes for Questions?

11. Work

12. Work = (net force) (distance object moves in the direction of the force)

13. Is this person doing work on the trash bag? YES NO

14. Work done by a system subtracts energy from the system Work done on a system adds energy to the system

19. As long as we account for the work in, is our ruler system a closed system (no energy in or out)? 19

20. If we believe in conservation of energy, then the work you do on a system shows up as energy of the components or work done by the system

21. Let’s Pause Two Minutes for Questions?

24. Work in = Work out If we ignore heat losses due to friction, then we can say that Work done on the system = work done by the systemor

25. FdFd = 1 2 2 1

27. Since d is larger than d , what do we know about the forces? 1 2

28. F1 = F2F1 > F2F1 < F2

29. F1d1 = F2d2

30. d is now smaller than d How do F and F compare? 1 2 1 2

31. F1 = F2F1 > F2F1 < F2

32. F1d1 = F2d2

33. With simple machines, there is almost always a tradeoff between force and distance

34. True or False? The F1d1=F2d2 we just used is the same as the “law of the lever.”

36. You push down on a lever, moving your end four times as far as the end with the rock moves. How does the force exerted on the rock compare to the force you exert?

38. Let’s Pause Two Minutes for Questions?

39. Pulleys

45. How does your pull in the previous slide compare to the force exerted on the washer?

47. Sometimes all a simple machine does is change the direction of the force you exert