Friday, October 5, 2001
Announcements:
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Group assignments are now posted - use UNC director if needed to contact
your group members
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Several "inactive" students have been included until I hear that they have
officially dropped.
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Your original real-world problems for Exam 1 are being re-graded and will
be returned Monday.
Student comments and concerns (from Minute Paper on 10/3/01):
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liked demos; they are helpful and fun - I will continue to use them
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liked example problems; they are helpful - Thanks.
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how do you know which equations to use - Decide which physics principles
apply
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review Chapter 5 problems: #27, 43, 47 - See solution manual
in the Tutorial Center
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work more problems in class - Will do so within reason
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work problems more slowly without skipping steps - I will try to strike
a balance
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demos are not helpful for test - I disagree; some test questions are straight
from class demonstrations!
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how to find the force of the ground on you when you jump - See Wednesday's
revised class notes
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if you jump up, does the Earth recoil? - Momentum is conserved, but v(earth)
is imperceptively small.
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please give overview of our "physics tool box" - See chapter summaries
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explain elastic/inelastic collisions - This is the focus of today's class.
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what equation is used for skateboard on a slope? - F = ma
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if a system has no change in kinetic energy, is momentum the same? - Yes,
but the converse is not always true.
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please post solutions to last year's exams - They are already on E-reserves
(see class notes from 9/28/01)
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count WebAssign only if it helps our grade - Will do; this is the new default.
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lab TA and grading - Lab grades will be adjusted if necessary so that all
sections are graded similarly.
Chapter 6: Momentum and Collisions
Review question: What is the maximum height a person can fall
without suffering severe injury?
(see also problem 50 in chapter 5).
Human acceleration
response data
Demos:
Momentum in one dimension - colliding carts
| |
m1
|
m2
|
v1i
|
v2i
|
p
|
v1f
|
v2f
|
KEi
|
KEf
|
|
elastic
|
m
|
m
|
0
|
v
|
mv
|
v
|
0
|
0.5mv2
|
0.5mv2
|
|
elastic
|
m
|
m
|
v
|
-v
|
0
|
-v
|
v
|
mv2
|
mv2
|
|
elastic
|
m
|
2m
|
0
|
v
|
2mv
|
4v/3
|
v/3
|
mv2
|
mv2
|
|
elastic
|
m
|
2m
|
v
|
0
|
mv
|
-v/3
|
2v/3
|
0.5mv2
|
0.5mv2
|
|
inelastic
|
m
|
m
|
0
|
v
|
mv
|
v/2
|
v/2
|
0.5mv2
|
0.25mv2
|
|
inelastic
|
m
|
m
|
v
|
-v
|
0
|
0
|
0
|
mv2
|
0
|
|
inelastic
|
m
|
2m
|
0
|
v
|
2mv
|
2v/3
|
2v/3
|
mv2
|
0.66mv2
|
|
inelastic
|
m
|
2m
|
v
|
0
|
mv
|
v/3
|
v/3
|
0.5mv2
|
0.33mv2
|
Ballistic pendulum - Example 6.6, why not use KE = PE ?
Newton's cradle
AstroBlaster/2 ball bounce
Problem: If you are lazy and want to close a door by throwing
a ball at it, should you throw a rubber ball or a ball of clay with the
same mass? Where should you aim?
Happy/sad ball demo
Assignments:
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HW6b (due Monday, October 8)
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HW7a (due Monday, October 8)
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Contact your group members and discuss ideas for your group's physics Web
Project