Friday, May 22,
2011
Announcements:
- Exam 1 scores for the multiple-choice section should soom be available via the onyen or
Connect Carolina websites.
- I will review the answers to this exam in class on
Monday.
- Summary of results from MC section:
- Average = 62%
(68% in 2009, 66% in 2008)
- Std. Dev. = 14%
- High =88% (100% in
2009, 92% in 2008)
- Low = 35% (29% is
expected score for random guessing)
- The total Exam 1 score will include 22 points for the
graphing question after this has been graded.
- Keep in mind that this exam counts for only 15% of your
overall course grade, and the lab, class participation, and homework
scores tend to boost most students' scores. However, the average
student improvement on previous exams I have given is 0.2% and the maximum
improvement was 13% averaged over the course of the semester, so this
first exam score is a pretty good indicator of how you will do on future
exams.
- Because the class average on this exam was somewhat
lower than I expected, you have an opportunity to boost your grade and
learn from your mistakes through Test Corrections:
- Provide written
justification (preferably on your original test papers - use back sides
or other paper as needed) to demonstrate that you understand each
question you missed (or questions you happened to get right because you
guessed correctly).
- Discuss your
explanations with someone who understands at least as much physics as you
do. Have this person sign the following statement at the top of
your test paper: “I am convinced that [your name] fully understands
the concepts and problem solutions for this exam.” Signed
____________
- Hopefully by discussing
your answers with another person, you will better understand and remember
the correct reasoning in case you are faced with a similar question on
the final exam or later in life.
- Submit your original
test papers with corrections by Wednesday, June 1 (the date of Exam2),
and assuming that your explanations are all correct, 5 points will
be added to your exam score.
- Answers to
multiple-choice questions on Exam1 (version 101):
- Version 101: B,
C, B, A, C, A, B, D, C, B, A, B, B, B, C, A, C, C, B, B, A, A. B, A, B, C
- Version 102: B,
C, B, C, A, C, B, A, D, B, B, A, B, C, B, B, C, A, C, B, A, A, B, C, A, B
- Answer key for
graphing question.
Assignments:
- HW7 is due Sunday at midnight.
- Speeding
Ticket Real-World Problem is due Wednesday morning.
Chapter
7 - Work and Energy
Fundamental physics principle - Conservation of Energy: E = constant
in the Universe
Energy can be in various forms: (Ch. 7 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!
Balls
rolling down 3 different tracks - How do the final speeds compare? Are
average speeds the same?
How can you find the maximum speed of a pendulum if you only know its length
and maximum angle?
Work: W = F*d = Fcos(theta)d
(vector dot product)
How much work is required to lift your textbook (m = 2 kg) to a height of 1
m? How much work is done in moving it horizontally a distance of 2 m?
What is the work done by your hands in lowering the book to its original
height? How much work is done by gravity in this case?
P. 7.10: How much work is done by a janitor pushing a mop 0.5 m with a
force of 50 N at an angle of 55 deg. above horizontal? How does the work
done change if the angle is increased to 65 deg.?
Power: W/dt (rate that
work is done)
Estimate the maximum power of a typical human. Express your answer in
both watts and horsepower (1 hp = 746 W).
Demos:
Dropper popper
Balls on tracks
Pendulum
Conceptual Questions
Minute Paper: Before leaving class today, be sure to answer the
following questions:
1) What is the most important thing you learned today?
2) What questions or concerns do you have?