Physics 8 — Monday, November 14, 2011
“I loved this reading. It was clearly and concisely written, easy to
understand, and fueled my motivation for physics. Can we read
more chapters from the Giancoli book? Sometimes when I read the
other textbook readings, I finish the chapter more confused than
when I started. It seems that just reading this one chapter from
Giancoli book not only helped me understand this chapter, but
cleared some confusion from previous chapters.”
“The layout and examples in this textbook are better than the
normal textbook we use.”
“I enjoyed this textbook more than the other one we usually read.
The diagrams are much more helpful.”
“I really liked this textbook. I’ve had a lot of difficulty getting
through the readings in the other textbook we’ve been using this
semester. This reading for tomorrow’s class was much clearer and
presented in a much more interesting way.”
Giancoli Chapter 9 (1st half)
“I preferred the way this chapter was more concise and had fewer
equations.”
“It was nice to read a new textbook: I liked the format of this
section.”
“I think this book is easier to get through. The examples are laid
out step-by-step and I don’t feel I miss out with giant leaps in
equations. I also feel the drawings were more informative with
their force directions.”
“I like this book a lot more than the other one because it had more
examples and was easier to understand. I found this reading to be
interesting because I can directly see how equilibrium relates to
architecture.”
“I really found this chapter very interesting. We’re finally talking
about areas of interest for architects. I also found this chapter
easier to read and therefore more informative. I understand these
concepts better than others. It also helped to reinforce a lot of
these concepts we touched upon in previous chapters.”
“I really enjoyed today’s reading because it was more of what I was
expecting when I originally signed up for this class- it seemed much
more architecture/structure focused. I really didn’t find anything
too confusing- I guess that means we put in a good base over the
past two months. I am looking forward to domes and arches, a
structural concept we have discussed heavily in my art history
classes and the main reason I took this course.”
I
This chapter’s material connects much more directly with
architecture than anything else we have done so far this term
I
But I think you can absorb it much better now than you could
have in the first week of the semester
I
It is also largely review of what we studied in Eric Mazur’s
“torque” chapter
I
What I really need to understand is whether you would have
found this book easier also for rotation, gravity, etc.
And if so, what do we do for the spring semester?
I
I
I
My present plan is to continue to use Eric Mazur’s book, but
to supplement it with 2 or 3 chapters from Giancoli
But if a few of you are willing to help me to do a side-by-side
comparision of a few chapters, I can reconsider.
This is still an option (but present plan is Eric’s book)
Spring term draft syllabus (part 1/2)
Spring term draft syllabus (part 2/2)
Homework #8, problem 11
Which is the correct equation of motion for the sliding block?
(Take x axis to point downhill. Take m and M to be masses of
block and wheel, respectively.)
(A)
max = mg sin 50◦
(B)
max = mg sin 50◦ − T
(C)
(m + M)ax = mg sin 50◦ − T
(D)
max = mg sin 50◦ + T
Which is the correct equation of motion for the wheel? Take m and
M to be masses of block and wheel, and R to be radius of wheel.
(A)
α=
R T
τ
=
I
MR 2
(B)
α=
(C)
α=
τ
R T
= 1
2
I
2 MR
τ
R (mg sin 50◦ )
=
1
2
I
2 MR
(D)
α=
τ
mg sin 50◦
= 1
2
I
2 MR
Corrected solution for HW8 #11 (sorry!)
max = mg sin 50◦ − T
α=
τ
RT
R (mg sin 50◦ − ma)
a
= 1
=
=
1
2
2
I
R
2 MR
2 MR
1
a 1
)( MR 2 ) = Ma
2
R
2
2
1
mg sin 50◦ = (m + M)a
2
mg sin 50◦ − ma = (
a=
g sin 50◦
(9.8 m/s2 )(0.766)
= 2.92 m/s2
=
M
47 kg
1 + 2m
1 + 2(15 kg)
T = mg sin 50◦ − ma = 68.8 N
my earlier solution is only correct in the limit m << M, so ax ≈ 0