w6d2

Wednesday, September 26, 2001

Announcements:

Exam scores are available online from the onyen website: onyen.unc.edu
Summary of results from MC section:

Average = Median = 12/20 = 60%
Std. Dev. = 15%
High = 17/20 = 85%
Low = 5/20 = 25%

This exam will be graded out of 90 points instead of 100:

Exam Score = (MC*4 + Car RWP + Orig. RWP)/90

Feedback for next exam:

How many of you needed more time?
How many feel that the length was OK if you had prepared better?
In terms of content, do you feel this exam was fair?
Would you prefer to have the RWP be take-home?
Which would you prefer:

20 MC (4 pts. each) = 80%
25 MC (3 pts. each) = 75%

Chapter 5 - Work and Energy

Fundamental physics principle - Conservation of Energy: E = constant in the Universe

Energy can be in various forms: (Ch. 5 focuses on mechanical energy)

Kinetic Energy: KE = 1/2mv²
Gravitational Potential Energy: PEg = mgh
Work: W = F*d
Elastic Potential Energy: PEs = 1/2kx²
Rotational Kinetic Energy: KEr = 1/2Iw²
Electrostatic
Electromagnetic (light, magnetic fields)
Heat (from friction, air resistance)
Sound (from collision, explosion)
Chemical energy (battery)
Nuclear

Problem-solving strategy: E(initial) = E(final) and define energy forms specific to situation

Conservation of energy can be used to solve problems when F=ma is too complicated or impossible:

Find the final speed of a child going down a slide that is 3 m high.

Using Newton's second law: mgsin(theta) - umgcos(theta) = ma

but F=ma can only be used for a flat (non-curved) slide so that acceleration is constant
Since the length of the slide was not given, we cannot even use F=ma.

Energy: mgh = 1/2mv² -> v = sqrt(2gh) Easy!

Ball rolling down 3 different tracks - How do the final speeds compare?

Bowling ball demo - Is any energy dissipated?

Work: W = F*d = Fcos(theta)d (vector dot product)

Power: W/dt (rate that work is done)

Assignments:

HW5b (due Monday)
Begin thinking about topic ideas for your physics Web Project