Chapter 6 : Work and Energy

Due: Wednesday Oct. 18

The Big Picture Ideas:

a.) Work Energy Theorem (you can always use it, with or without friction)

b.) Conservation of Mechanical Energy (can only be used there are no frictional forces)

c.) Springs (forces from springs and elastic energy)

Required:

Put your name and Lab day on your assignment

Questions: 3, 12, 16, 17, 22

Problems: 3, 7, 20, 25, 37, 41, 51, 66

Reality Check

Dr. H’s Reality Check

You and your friend have a disagreement about who can go faster on their skateboard. It would be nice to have a definite way to figure it out with out using a stopwatch and a meter stick. You decide that you can find out who is going fastest by coasting up a ramp. Explain how you know who is going fastest and the physics concepts behind it..

Suggested:

Questions: 1, 4, 7, 11, 12, 18, 24

Problems: 2, 8, 22, 24, 29, 36, 54, 58

Answers

2.) a.) 1080 J b.) 5400 J (This is the work required only by the person to do what is asked. Since it moves at a constant velocity the net Work is zero, i.e. the work energy theorem)

8.) Work done by the 12 N push = +169 Joules

Work done by friction = -169 Joules (constant velocity so Wnet = 0)

Work done by weight = 0 Joules ( perpendicular to displacement)

20. -4.6 x 10⁵ Joules

22. 44 m/s

24. 2.25 times greater

36. 49.5 m/s

54. 0.31

58. 25.5 seconds

66. 1200 Watts

2. Three different cars go down the middle track above. They each start from rest. Cart A has a 1000 kg mass, cart B has a 1500 kg mass, and cart C has a 2000 kg mass. Which cart is the fastest at the bottom,

A B C Same Need More Info

Why?