Archived questions and answers for 2010, Fa 98
First week
To stimulate some questions, here's one for you.
Who is this?
First, some "admin" questions:
>hi Mr. Pollock.after attending today's recitation i know i'm going to
>need a lot more help. [...] 1. i want
>to know how i can find out about other t.a.'s office hours 2.if there
Hi!
A list of all the TA's office hours (and mine) is on the course web
page (also here ) . Feel free to see
anyone you want to for help. I'm also trying to arrange for some
undergrad physics majors (SPS students, "society of physics students")
to provide a little free tutoring too. Don't know if that'll fly, I'll
keep the class informed.
>is a list of (really good) tutors i can find [...]
The physics office keeps a list of graduate students who are interested
in tutoring (for $$) I don't know how current it is, and I know for
sure that no one tries to "rate" them. TA office hours are free, so
probably at least start with that. Tutors can be great, or worthless,
you might want to ask around a bit for recommendations (I'm afraid I
don't really know anyone to recommend). Also, I'd urge you to find some
other fellow students in 2010 who you think you could form a study
group with, you'll be surprised how helpful that can be. Even if
everyone is a little lost, you'll find everyone is often lost in
different ways, and you can often piece together a lot! (Keep the group
small tho, 2 or 3 is good, more than that and the social aspects starts
to make it kind of ineffective physics-wise.)
>>[signed ...] p.s. where is the gamow tower?
The Gamow tower is the 11 story building next to where the lecture hall
is. Just walk down the hall, I guess it's west (?) till you see the
physics office on your left, and the elevator to the tower on your
right. If you step outside the building, it's real easy to spot!
Thanks for the questions - I'll see you in office hours sometime!
Steve P.
A CAPA comment:
I've gotten a complaint about HW1, question #2 that people are
inputting the "correct answer" and CAPA says its wrong. CAPA is *not*
making a mistake here, but the issue is a bit subtle. In this problem,
you must think about SIGNIFICANT FIGURES. There's a nice discussion in
Giancoli, Chapter 1. When you add numbers with differing amounts of sig
figs, you have to think a little about how many digits your final
answer should have (*including* exactly how many trailing zero's to
input!)
You can tell immediately if this is your problem, because CAPA will say
something like "Please adjust significant figures, tries 0/20." (If
you've previously entered some wrong answer, that tries number will
differ, but it WON'T increase if your error is simply sig figs!) So,
think about exactly how many digits you *should* be entering for this
answer. (It shouldn't matter if you try to enter your answer in m, cm,
or mm, as long as you get the answer correct, and enter the units
properly, with a space between the number and the units)
Sig figs are especially important whenever you're doing labs (in this
course and anywhere else!)
CAPA Question
I am having trouble answering the sixth problem. The question reads,
"How many seconds are there in [..] hours? " I believe the answer to be
[..] sec. When I enter this into CAPA it says units required. It is
my understanding that the sec. are the units. If you could help me out
with this problem that would be great.
The standard SI abbreviation for seconds is "s", not "sec".
Admin Question
[How does CAPA keep track of homework points? Do mistakes or tries
count against you?]
CAPA does record and save *all* your inputs, but in the end your
grade is based only on the *total number you ultimately got correct* by
the deadline. (Mistakes or tries do not ever count against you, except
of course if you exceed the maximum number, then CAPA won't let you try
anymore!) I generally never look at individual tries or mistakes
unless someone has a specific question.
Concept Test Question
>on the nonsense exam we took
[NOTE to 10 oclock lecture people - I added a second example concept
test in the 12 oclock lecture - a big truck and a little car collided
head on, and the question was to compare the force of the car on the
truck to the force of the truck on the car. The correct answer is that
the forces are EQUAL in magnitude, but most of the class "voted" for
"force of truck on car is greater", even after discussion with each
other...]
(Wasn't meant to be nonsense, but it was getting awfully far ahead of
things!)
>today there was a question about the collision of a big truck with
>a little car. i was wondering why speed of the vehicals had nothing to
>do with the force exerted on each other, because my aristonion thought
>led me to believe it would matter.
Well.... I didn't exactly say that the speed had *nothing to do with
it*, the higher the speed, the worse the crash, naturally. The forces
will be bigger in a higher speed crash, as will the accelerations. But
the point of the ConceptTest was - how do the two forces (car on truck
and truck on car) compare *with each other*. The amazing fact of
Newton's third law is that the forces are equal and opposite. They
might be equal and opposite and BIG, or equal and opposite and SMALL,
that does depend on the speed. But no matter what the speed, the forces
are *always* equal and opposite.
Please bear in mind - we're not going to get to Newton's third law for
about 3 weeks, and we'll be learning a lot of useful and relevant stuff
before we get there. (Like, what *exactly* acceleration means, and what
*exactly* force means, to a physicist!) So please don't worry if this
example seemed pretty odd right now!
>i really don't know anything about
>physics or any equation which might explain force (granted i haven't
>finished my reading)
Not to worry - this is Ch. 4 stuff!!
>but it seems to me if one car was moving fast and
>another car wasn't moving at all the fast car would exert more force on
>the car that wasn't moving
Nope! It's pretty counterintuitive (that's what I meant about having
to give up some Aristotelean intuitions!) but if one car is going fast,
and another is stopped, when they hit they really do always exert the
SAME force on each other. We'll be doing demos about this soon, and
trying to build up to the concept, (the "test" today was just for
fun...) but the bottom line is that if object A pushes on object B,
then object B will push right on back on A with the *same* force.
>because the motionless car only had the
>force of being in the way.
Let me leave this sentence alone, when we get to Ch. 4 dig this note
out and see if your questions and intuitions have changed at all..
>i don't know if you'll understand what i'm
>trying to say, but i'll hope anyway. thanks for all your help!
Technical CAPA Question
>I am enrolled in Phys 2010 200 and just tried out the CAPA. What a
>neat way to do homework. Just one quick question. Is it enough to just
>hit the SubmitAllAnswers button after revising some answers? Does this
>automatically transmit all correct answers ?
Glad you like the CAPA system, I'm pretty psyched about it too!
If I understand the questions, the answer is yes (to both). It should
be the case that when you hit "submit", *all* the entries that you have
filled in get submitted and checked at once. So, you can go through and
fill in all the entries you want, and then hit submit to save yourself
a little "web access time" (because every time you hit submit it has to
completely reupdate the whole page) If any entries are left totally
blank, they are ignored (so, the submit doesn't count against your
total number of allowed tries for the blank ones)
Once you have gotten an answer correct, CAPA won't even *let* you try
to reenter anything for it. So you can never accidentally "erase" any
problem that you once got right. (To verify the scores that CAPA has
stored for you, just go to "Term summary" at the top of the CAPA input
page, and you'll see your scores for each problem from the whole
semester) Once you're satisfied you've gotten all the problems you can
get, there's nothing else for you to do - CAPA has already stored your
score.
See you in lecture,
Second week
An "admin" question
>hi Dr. Pollock - what extra help or tutoring is available?
Hi!
A list of all the TA's office hours (and mine) is on the course web
page (also here ) . Feel free to see
anyone you want to for help. The TA's office hours + mine are the
*primary* source of out-of-class help. As I mentioned in lecture last
Friday, we now have a couple other sources that are still free - I have
gotten a grader to volunteer to run a "tutoring session" Thursday
evenings at 7PM in G-031 (he'll shift to Monday at 7 on exam weeks
only) We also have a "computer-lab tutor" (who can also answer homework
questions) Fridays at 4, in the computer lab next to the Math/Physics
Library. The Society of Physics Students is arranging for a group of
undergrad physics majors to also run evening tutoring, maybe at the
same time and place as the Thursday evening one. (Not yet decided or
arranged)
Beyond that, I can suggest getting together with other students from
class, forming a small study group (I recommend groups of 2-4) There
are physics tutors available too (physics office keeps a list), mosly
physics grad students looking for extra $$, but I hope you can take
advantage of enough of our free resources you won't need to pursue this
option! (I think some of the residences also have physics tutors)
A lecture question
>Dear Steve:
>In today's 10 am lecture, you showed Xf = Xi + v(avg)*delta t
>is this equation correct when velocity or acceleration is constant???
The equation
Xf = xi + v(bar)*Delta T
is always correct. It comes from the *definition* of vbar,
vbar = Delta x / Delta t
(It's true whether or not a is constant) However, it's not very USEFUL
unless v is constant!
It's not useful if v is not constant because then you don't know what
vbar is! Even in constant acceleration problems, this formula is not
very useful, because v is changing all the time, and you may still not
know what vbar (the average velocity) is. You can usually
figure it out, but it's almost always more useful to just use one of
the "specialized constant acceleration equations"
In the book, the formulas in the box on P. 28 are the ones you're going
to use all the time. They are all only valid if a is constant. (By the
way, that means the formulas implicitly include the case a = constant =
0, i.e. if *velocity* is constant! It's a "special case", where a=0)
A CAPA question
>I was writing to ask you if I am supposed to know the exact distance in
>problem #2 of set 2. I thought you calculate distance/time to figure
>out average speed. My problem is, I cannot figure out exactly what the
>distance is just by looking at the graph in problem #1. If there is
>another way to figure out this problem, please help me out.
There is not another way - you just have to look at the graph, and try
as carefully as you can to figure out the distance. (It'll be more
accurate if you have your hard-copy homework in front of you, a little
pocket ruler might help you too) CAPA has been programmed to give you a
lot of leeway on this one, since you do have to get the numbers off a
graph, so there's some "human error" allowed for.
A study question
(I've gotten a few questions from people looking for some extra ways to
study, prepare for exams)
Two obvious things *besides* doing CAPA, reading and thinking about the
book and lecture notes carefully, and working through the book's
examples.
First, working a few of the odd-numbered problems at the end of the
chapter is a really good idea - especially in those sections where you
feel a little unsure. They're well organized, section by section, with
difficulty ratings on them. (I suggest odd numbered, because then you
can check that you got it right in the back) Also, you should look at
Giancoli's web site. There's a link to it about halfway down my 2010
page. It's a fantastic site, with *lots* of extra problems, and a sort
of CAPA-like system for submitting answers. He has various types of
problems, qualitative and quantitative, including special MCAT style
questions.
Final comment - reading the text and lecture notes, attending lectures,
and doing the HW is not enough. You have to understand the material!
Here's a test of "understanding": If you can explain the material in
words to someone else, without refering to the text, you probably
understand.
Week of Sept 8
A study question
>Hey there! I just caught up on all the readings for 2010 through
>chapter 3 this weekend and chapter...
>I was wondering about the problems at the end of the chapter...do we
>have to do all of the problems, or is there a list of the "essential
>ones" somewhere.
You certainly don't have to do ALL of the problems at the end of the
chapter. (You don't have to do ANY if you're confident you're
understanding the material) But I do suggest that people try maybe a
couple from each section (do odd-numbered ones, so you can check
yourself in the back of the book), especially any sections you're
feeling unsure about. If they go fine, you're probably set. If you
have troubles, then do some more. There's no list of recommended ones,
they're all decent and relevant. Giancoli indicates the "hardness
level" (I for easy, III for pretty hard) on his problems - Use I-types
to test your basic knowledge, II-types to make sure your knowledge is
thorough, and III-types if you want to see if you can go beyond the
straight issues, and have really understood the physics. Exam
questions will be a mix (but obviously not many III types - not enough
time!) If you want still more problems, check out Giancoli's web site
(there's a link partway down the 2010 home page) - he has more
"CAPA-like" problems, and some nice exercises for each chapter (plus
MCAT-style questions too!)
Hope this helps -
Week of Sept 14
Monkey Business
>i wanted to ask you if the speed or velocity of the bullet in your
>demonstration matters. it seems to me that if you shot the bullet faster
>it would travel farther in a straight and go over the monkey's head. like
>throwing a ball vs. shooting a gun. am i way off? thanx,
The velocity does matter a little, but only in the following sense. The
faster v0 is, the quicker the bullet gets to the monkey, so the closer
to the starting height they hit. If v0 is slower, they both have more
time before the collision, and fall farther, and they hit closer to the
ground. So, velocity does effect how high they BOTH are when they hit.
But no matter what the velocity is, they WILL hit! If I take a high
powered rifle, it'll hit the monkey right near where the monkey
started. If I lob the ball more gently, it might hit right near the
ground. But *No* speed will make it go over the monkey's head!
Of course, I'm assuming the monkey lets go the instant the bullet is
fired. I'm ALSO assuming, as was stated in the concept test before the
demo, that the monkey was "within the range of the gun", which means
basically initial velocity IS high enough to at least reach the
horizontal position of the monkey. Clearly, if v0 is TOO small, the
bullet will plop to the ground nearby and never hit the monkey.
CAPA 4 questions
>...[Question was asked, is it T/F that]: If object's speed doesn't
>change, no net force is acting on it.
Think about an object moving around a circle, with constant speed. Is
there a change in velocity (vector)? If so, what does that say about
acceleration, and thus net force?
>The one other statement in this question is unclear to me: "A truck
>initially moving at a constant velocity will slow down unless a small net
>force is applied." Should we assume this trunk is moving on a (friction)
>road or moving on a frictionless surface?
No need to make assumptions. The key word in the statement is "...
unless a small NET force is applied". Net really means net, i.e. the
sum of ALL forces in the problem (including friction, etc) If the NET
force is small (but not zero), what does Newton's 2nd law tell you
about the motion?
>Also, I read in Giancoli that even if a force is acting on an object, it
>doesn't necessarily mean that there will be motion, I understand that.
>But if there is NET force acting on an object, it would definitely mean
>that the object will change it's velocity, right?
Precisely!!
Another CAPA 4 question
> I have a quick question about entering degrees in to the copmputer for
> CAPA. ( I looked around but couldn't find anything specific for
> degrees)
I believe the unit for degrees is "degrees", and also "deg" should work.
Week of Sept 21
CAPA question
>My question is if a pulley system is set up like Question 7 on this
>week's CAPA assignment, how would I calculate the T3 value (the tension
>of the cord between the ceiling and the actual pulley itself)?
>I think it would equal to the mass of the weights below multipled by
>g. Am I on the right track?
Well, not quite. I would say that if you focus your attention on the
upper pulley, the sum of all forces on it must be zero, because it is
clearly not accelerating. What are those forces? There's an upward
tension (T3), and TWO downward tensions from the ropes on the two sides
(T1 and T2), and both of THOSE must be equal to each other, because
tension in a single rope is always the same everywhere along the rope!
It's not true that T1 = M1g, or T2 = M2g, (otherwise, they wouldn't
accelerate!) so it's surely not the case that T3 = M1 g + M2 g.
Maybe that's enough to get you started thinking further about the
problem? Let me know..
Another CAPA question
>On problem number 4, would the acceleration of the girl be equal to the
>force of gravity plus the downward acceleration of the
>elevator(9.8+2.2=12)? THus, the girls weight would be that
>acceleration times her mass?
1) Acceleration cannot equal force. (Units aren't right)
2) Acceleration of the girl is GIVEN, you don't have to add anything to
find it! She IS accelerating down at 2.2 m/s^2.
3) Use Newton's 2nd law on the girl. Think of all the PHYSICAL forces
acting on her, add them (watch signs!) Set the sum equal to ma...
Finally, better decide what force in your problem is her apparent
weight...
If this doesn't help - better come see a TA (or me) during office hours!
>Also, I don't understand what the strange
>u to the s and u to the k stand for?
"mu" is a greek letter, used to represent friction. In the book, and my
notes, and your capa handout, it is *written* in Greek, so I can't
describe what the symbol looks like in this email. On the web, I don't
have any Greek fonts available, hence the weird "mus" and "muk". The
"s" or "k" afterwards is a subscript, "static" or "kinetic". So "mus"
is "Greek letter mu, subscript s" or in otherwords, the "coefficient of
static friction". "muk" is "coefficient of kinetic friction". See
Giancoli's section on friction to see how to use those coefficients -
we haven't done this in class yet (we'll do it next time)
Hope this helps!
Study issues
>I thought the test however was very conceptual and thus I had to think
>much more than expected.
I think that's a good thing :-)
>I just signed up with a tutor but I was wondering if there is any way
>I can drill myself on the concepts. I already print your lecture note s
>as well as attend every class but sometimes I have troubles grasping
>the mater ial. Your office hours are swamped and I don't want to add to
>the chaos.
A tutor is an o.k. idea (depends a lot on the tutor), but I urge you
*not* to end up letting the tutor "explain" everything, especially
homework problems. Be sure you've worked a *lot* on a problem before
you go seek help on it. And then, when you get help, try to get the
minimum amount of help needed for you to continue working on your own,
and no more. Turn's out there's not nearly as much value gained from
having someone explain a problem to you as having worked it out
yourself... I do encourage you to visit TA's office hours (you can go
to *any* of them, not just your own TA) every week, many of them are
much less crowded than mine. Don't be shy about adding to the chaos in
mine though, if you show up a bit early (or near the end) my office
hours tend to be quieter, and Monday is often a lot quieter than Wed.
Are you in any kind of study group? If you can find another person (or
a couple of people) who'd like to just get together somewhere, maybe
somewhere middle in the week, and just work on CAPA's together, that
can be very helpful for some people. Having to explain a problem to
another student who didn't get it does *wonders* for your own
understanding of the concepts!
In terms of grasping the material - I can only suggest trying to
carefully read Giancoli *before* the lectures, then attend the
lectures, and if the concepts are still unclear, go talk to a TA, or
me, or even another student and try to hash it out. You should
understand every concept test, in the end, even if you didn't get it
when you first heard it. (I put a few extra CT's on the web to help you
practice - Giancoli has lots of "questions" at the end of each chapter,
before the problems, which also help you decide if you're getting the
conceptual aspects)
If it looks like there are issues on the "straight formula" problems
too (problems 1-9 on the exam) - working a few extra problems (like,
level I odd-numbered Giancoli end-of-chapter problems that you can
check in the back) might also be pretty helpful.
Good luck, feel free to come talk with me if you feel you need more
direct help!
Week of Oct 5
Exam II
>
>I was wondering when the review session for Physics 2010 for our next
>test will be. I looked for possible information on your web page but
>couldn't find it. If possible please let me know when the review
>session will be held. Thank you.
I'll be in G030 Monday evening (next week), 6-7 PM. There'll be a
graduate TA across the hall from 7-8 that same night. Information is now
posted at the "late-breaking announcements" link at the top of my page.
>... what will be covered on the second exam?
Exam II covers Chapters 1-6.3. The obvious emphasis on this
exam will be new stuff since exam 1, meaning ballistics, forces and
Newton's laws, circular motion, gravity, and work.
When I try to get the old exam from the bottom of your weg page it
says the file is not found when I click on the exam II. Is there
another way to get this old exam or any other old exams?
The prof who taught that course has gotten those links back up and
running, sorry for the inconvenience!
Week of Oct 12
>I was wondering if the undergraduate help group was tonight. I
>remember you announcing to the class that during exam weeks they may
>not hold a group.
As far as I know, SPS meets every Thursday. (It's the grad student,
Justin Freeman, who shifts his Thursday evening session to Mondays on
exam weeks) See my "office hours" link off the main page, I list room
numbers and times for these evening hours as well.
Humor, from a student
Postulate 1: knowledge is power
Postulate 2: time is money
As every engineer knows:
Power = Work / Time
Since:
Knowledge=Power and Time = Money
Then:
Knowledge = Work / Money
Solving for money, we get:
Money = Work / Knowledge
Thus, as knowledge approaches zero, money approaches infinity regardless
of work done.
Week of Oct 19
Question about collisions
>I think I am still a little confused about elastic collisions. You
>talk about a car accident as an elastic collision.
Careful! Car accidents are usually NOT elastic. Elastic means that
kinetic energy is conserved, so no friction or noise or dissipation of
any kind. Car accidents are usually *inelastic. However, elastic or NOT, they
do conserve momentum! (Elastic has nothing to do with momentum conservation,
elastic tells you about energy conservation)
>Let's say for instance that a car hits a building. Is the collision as
>a whole an elastic collision?
Almost assuredly not. A superball bouncing off a building is elastic (energy
before and after is the same), a car hitting a building will probably stick -
that's completely inelastic
>This would mean that momentum would be conserved in the system -
Momentum is always conserved in collisions, elastic or no...
>but you can't see the building move.
You can't see it, but it does. The whole earth moves!
>I suppose that you could conjecture that the earth moves as a result
>of the collision between the car and the building.
Yup, precisely!
>But how do you know this, and how would you measure or calculate it?
I *know* it because Newton's second law tells me it's true. And
Newton's second law has been so incredibly and deeply tested in so many
cases - big things, little things, fast things, slow things, energy
conserving things, cases with friction, you name it. And it's ALWAYS
true in all those tests. So, I conclude with extremely strong
confidence that it would continue to be true even with something the
size of a planet. NASA scientists make use of conservation of momentum
between small objects (satellites) and entire planets frequently, and
they are able to reliably guide the satellites where they want them to
go!
>I may have misunderstood an example you used. I spent all weekend
>puzzling about it - and how I could possibly guess right about this on
>an exam!
I think it's good you're spending your weekends puzzling over physics
:-) The example of a car running into a building is a particularly
DIFFICULT one for conservation of momentum, precisely for the reasons
you're puzzling about - how can you *measure* the momentum of the
earth? (Calculating it is easy - you know its mass, find its v!) I
would be unlikely to ask about a car crashing into a brick wall in a
conservation of momentum chapter for precisely that reason - I prefer
examples like crashes between a small super-ball and a big basketball
(for example) where the masses are different, but still "normal sized"
so that you can build and maintain some reasonable intuitions about
momentum conservation.
In the car crash with a brick wall example, the wall (and earth
attached to it) must start to move with the same mv as the car had
before. Since m is so huge, v is immeasurably tiny, and momentum
conservation is not an especially USEFUL concept in this example, but
it's still technically valid.
Does this help any? It's kind of hard over email - come visit me, or
any TA during office hours, and maybe you can ask more questions and
clarify everything you're puzzling over!
Good luck,
Steve P
Question about concept tests online
>Prof. Pollock, why can't I access the in class questions from my home
>computer through www.colorado.edu? Do I have to be using a computer on
>campus only?
I have set the in-class questions pages to be readable only by people
who are logged in to a browser from a *machine* with a ".colorado.edu"
address. (So, either on-campus machines, or people dialing directly
into the CU modem pools) If e.g. you're coming in via AOL, I think you
won't be able to see the concept questions. The reason is that many of
the concept questions come from a copyrighted (<-spelling??) book, and
I don't feel comfortable making them openly accessible on the web.
(Plus the publisher would probably sue the pants off me) Sorry for the
inconvenience, guess you'll just have to use a campus machine to print
them out at some point (or dial in directly to CU if you can?).
Practical uses of 2010
> Prof. Pollock,
>I was just wondering if you caught final jeopardy on Wednesday night.
>I was flipping through the channels and saw the clue of "physics
>terms." The question was --Another term for "projectile motion" or to
>be "very angry." It took me about a second to guess balistics. NO ONE
>GOT IT RIGHT! Some guy even put velocity and I don't even know what
>the others got.
Nice to know this physics stuff has some practical applications :-)
Giacoli's website
>Prof. Pollock, have you used the companion website to the Giancoli
>text? I was browsing through it and there are many different
>self-tests that students can do and submit online and then get the
>results back. Do you think a site like this would help in preparing
>for the exams?
>
>The address is http://www.prenhall.com/giancoli
Yup, I sure do know about it, in fact I have a big old link to it on my
website! I think it could definitely be useful to study with. If you
have the time and want more problems, I think that's pretty much an
excellent way to prep for exams. (If your time is more limited, I think
the odd problems at the end of the chapter in Giancoli are plenty fine,
though!)
Sometimes I find Giancoli's site to be a little too "slick" - his java
stuff sometimes crashes my browser, and certain problems don't appear
quite right on my screen (the numbers take a long time to load), and if
you access it with a slow modem it takes a pretty long time to load
some of his pages. So, I haven't been agressively advertising the site,
but if it works on your machine I think it's definitely worthwhile, and
pretty entertaining in spots too! Lots of nice applications, java
applets, links to related sites, it's pretty comprehensive. Thanks for
spreading the word!
While we're on the subject - have you noticed all the other links at
the bottom of my 2010 main page? Check them out sometime, some are
useful for 2010, most are just for fun...
Week of Oct 27
CAPA set #10 hw questions
>On capa number three, is the flywheel moving at a constant velocity of
>83 rev/min(which would make the tangetial component of linear
>acceleration zero since it is at a constant velocity).
On capa #3, you should assume that it's still referring to all the
givens from problem 1. (So, the flywheel IS accelerating angularly,
problem 3 is just asking for the value of the tangential component at
some particular time...)
>Also, on capa number five you say to use equation 8-14, so to solve for
>the angular acceration wouldn't you take FR/mR^2. because when i try
>that i don't get the right answer.
Are you sure you're asking about number 5? You don't need torques for
that one. I bet you're referring to number 6 and/or 7? Your formula is
*almost* right, but you're dividing by mR^2, which isn't quite right.
What is the moment of inertia of a "solid cylinder"? (It's in the table
of moments of inertia in Gaincoli)
>Thanks.
You're welcome!
Week of Nov 2
Question about exam III
>Hi, I just want to know which chapters we are responsible for the exam
>III. The course syllabus says that it will cover ch.7-10, but how about
>ch.6?
The syllabus is not quite up-to-date. Exam III, like all exams in 2010,
is cumulative (you're responsible for everything we've been doing all
semester, since it really all builds on itself) But the primary focus
is on the new material since the last exam, which means Ch 6, 7, 8, and
9. That's the cutoff (Mon, 11-2 lecture) for new material, so fluids
will NOT be on this midterm. Statics (torques, etc) *is* covered on the
exam, basically any stuff that's due on CAPA this week is legitimate
for this midterm.
Hope that helps, talk to you,
Cheers,
Steve P.
Week of Nov 7
Question about grading
> ... Will you be dropping a CAPA?...
Yes, I will drop your lowest CAPA set of the semester.
Question about exam III
>Hi, I just want to know which chapters we are responsible for the exam
>III. The course syllabus says that it will cover ch.7-10, but how about
>ch.6?
The syllabus is not quite up-to-date. Exam III, like all exams in 2010,
is cumulative (you're responsible for everything we've been doing all
semester, since it really all builds on itself) But the primary focus
is on the new material since the last exam, which means Ch 6, 7, 8, and
9. That's the cutoff (Mon, 11-2 lecture) for new material, so fluids
will NOT be on this midterm. Statics (torques, etc) *is* covered on the
exam, basically any stuff that's due on CAPA this week is legitimate
for this midterm.
Hope that helps, talk to you,
Cheers,
Steve P.
CAPA 12 question
>..why would fisherman floating on water instead of sitting in the boat
>cause no water level change? I was thinking that water level would
>fall because the fisherman has a life jacket on in water, thus
>displacing less water?
The fisherman is wearing the lifejacket in the boat and in the water;
her *weight* is the same wherever she is. When she's IN the boat, she
must be displacing an extra amount of water equal to her weight
(Archimede's principle) If she's in the WATER, she STILL must be
displacing an extra amount of water equal to her weight (She's not
sinking in either case) Either way, she displaces the same weight of
water (equal to her own) Make sense?
Bottom line: to hold something up in a fluid, like a person, you need a
Buoyant force equal *exactly* to the person's weight. So, you need to
displace exactly the person's weight of water. It doesn't matter what
their shape is, where they are sitting, it they float on their back or
in an upright position - if they *float*, they displace the same amount
of water.
>Also, this wasn't a homework question, but do the three different
>flasks' surface (like the three in problem 8) experience different
>atmospheric pressure due to the different size of their openings?
Atmospheric pressure is about 1.0E5 N/m^2 (at sea level), no matter
what the size, shape, or orientiation of any surface. All three flasks
experience the exact same atmospheric pressure. Opening size is
irrelevant (as long as it's not so small that frictional or fluid vapor
type forces begin to play a role) Of course, a flask that's necking
down has less area up at the top of the fluid, and so feels less direct
FORCE (force = pressure *area), but that's a different question...
Circular motion questions
>Dr. Pollock, I'm confused about total linear acceleration. According
>to the book on page 212, "The total linear acceleration of a particle
>is the vector sum of two components: a=atan + aR."
>
>According to a CAPA problem on set 10, #5, "calculate the tangential
>angular acceleration, alpha, and this will yield the linear accel. of
>the bike."
>
>Apparently I'm missing some sort of distinction between these two
>problems, but I can't figure out what.
There *is* a (big) distinction between the two problems. When Giancoli
says the total linear acceleration is the vector sum of two components,
he means the linear acceleration of a *spot* on a rotating wheel. This
is only valid for an object turning about a fixed axle, with an angular
speed varying with time in some way (like, it's accelerating) So
Giancoli is talking about the linear motion of a point on a fixed,
rotating wheel.
The CAPA problem was about the linear acceleration of the BIKE. The
bike is not rotating at all! In other words, you're being asked about
the linear acceleration of the *middle* of the wheel, which is simply
moving in a perfectly straight line all the time. It only has one
component of acceleration. (Which is given by alpha*R) Giancoli has a
little section about the speed (and acceleration) of bikes, or in other
words the *middle* of a rolling wheel - around P. 216. Check it out,
and let me know if you're still confused about this one (It's kind of
an important point!)
Week of Nov 14
HW 13 questions
(There were a couple of questions about the pendulum in the rocket)
Rather than try to use a formula, I might argue you can intuit many of
the answers. Just think about how you would FEEL. E.g., in a rocket
which is accelerating upwards, just like in an elevator accelerating
upwards, you feel extra heavy. You feel like gravity has increased. We
say you feel "extra g-forces". So, the pendulum feels like that too -
not as though its MASS has increased (because you don't feel denser!),
but as though "g" has increased! Similarly, if the rocket accelerates
downwards, you feel lighter, as though "g" has decreased. (In reality,
it has not - even in the space shuttle, the force of gravity on the
astronauts is very nearly the same as it is on earth, just a few
percent less. But because the shuttle is in free fall, the *effective*
gravity feels like 0, they feel weightless, and pendulums would just
hang there without ever falling or oscillating)
Question about grading
> ... Will you be dropping a CAPA?...
Yes, I will drop your lowest CAPA set of the semester.
Two questions about classroom discussion of waves
>Hi! You mentioned this morning that transverse waves cannot
>occur in a water medium, but aren't ocean waves transverse because
>particles in the water move up and down?
Yes indeed, you're quite right - ocean waves are transverse - but
they're on the surface where there are both restoring forces and
transverse connections which are required to sustain a transverse
wave. (When the water level rises at one spot, the fluid water sees
lower pressure on its sides, where there is air, and so is free to go
there and thus raise the level beside it. If the wave is a trough, the
water on the sides sees the trough and again wants to flow down and
fill that. So, you get a "sideways" connection that can sustain a
transverse wave). But well below the top, this won't happen. You don't
see much in the way of transverse waves very far *under* the surface.
If you swim in the ocean, even when there are huge waves on the
surface, if you can dive more than a few meters down (and, do this far
away from the shoreline) you barely notice them. So I guess gasses can
hardly sustain transverse waves at all, fluids slightly more so but
still barely (much better at their surface than in the volume), and
solids best of all, anywhere.
>Also, the demo in class of the wave occuring on that long piece of
>metal, why was there reflection at the bottom when there was no tension
>there? (If I create a wave on a string of some sort, the wave would be
>lost at the end that's not anchored).
We'll talk a little more about this soon when we talk about musical
instruments - you *can* reflect a wave off an "open" end. You said that
if you created a wave on a string the wave would be "lost" at the end
that's not anchored, but that's not true. If you wiggle a rope, or a
slinky, you'll see a reflected wave EVEN if the end is not anchored!
The unanchored end tends to be pushed by the wave, and then the
momentum of the end point makes it move PAST the equilibrium point, so
that it then wants to come back, which starts a new wave heading back
the other way. (Think about energy conservation - if you have energy in
a wave traveling down a string, and it reaches a free end, where would
the energy go? It might dissipate into heat, but without losses the
*entire* wave would have to reflect!)
Here's the scoop with reflections: if you have a wave traveling along a
rope and it reaches a fixed spot, the wave will reflect *upside down*.
(An "up" pulse on the string reaches the anchor, and then turns to a
"down" pulse heading back.) But if the wave travels and reaches a loose
end, it will relect "rightside up" (An "up" pulse reaches the loose
end, flops it extra far, and then an "up" pulse heads on back)
Some instruments (like a flute) have pipes that are basically closed at
both ends, so you make waves that reflect back and forth off the closed
ends (like a string fixed at both ends) But some instruments (like
certain organ pipes) have open ends (at least at one end) and the wave
that's created inside the pipe (a pressure wave) reflects back and
forth, even off the open end (!) Just like a string floppy at one side,
or the torsional wave demo reflecting off the loose end. Weird, eh?
Great questions!
Week of Nov 23
Some questions from the heat and thermo material
>Hi! I'm having trouble with the mixing water and ice problem. I followed
>and understood the similar example problem in Giancoli. However, when I
>used the similar method to calculate the equilibrium temperature of the
>CAPA problem, I obtained a negative temperature. That does not make
>sense; it is expected the eq. temp. should be between the two initial
>given temperatures (0 and 91 degC for my set). What am I doing wrong? (I
>checked that all the units and algebra were indeed correct).
You are right that equilibrium temp has to be between the two initial
ones. I'm not 100% sure which problem you're referring to. If it's #4,
there's a sort of a trick - if you pour water onto ice, you might not
melt all the ice! If that's the case, the water you pour in will cool
down to 0C, and some ice will melt (but not change it's temperature, it
stays 0C). You have to check for this special case, and I think that's
what's going on in this problem. Let me know if this doesn't help you.
>Also, if thermal expansion expands all sections, why does expansion not
>occur in all directions (to fill a hole cut from an object)?
Did you read Giancoli's conceptual explanation of why holes expand,
rather than filling in? Think about what a thin RING does when you heat
it. All the parts of the ring expand, expanding the ring, making the
hole BIGGER. A hole in a metal plate is very much the same thing. Keep
thinking about it till it makes sense to you!
>Is water the only exception to the rule that objects expand when heated?
First of all, water is *only* an exception to this rule in a very
narrow range of temperatures quite close to 0C. (0-4, I believe)
Secondly, it's not the only exception. E.g. if you heat a rubber band,
it will tend to shrink.
Example of anonymous email
From anonymous@boulder.Colorado.EDU Tue Nov 24 09:48:47 1998
X-Sender: anonymous@colorado.edu (Unverified)
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
To: Steven.Pollock@colorado.edu
Dr. Pollock,
I was on the web page looking at the in class questions. When I click the
solution box, nothing happens. This happens for the regular in class
questions and the extra in class questions. Is this the new format, or do
I have a computer error?
Dear anonymous: I can't email you a reply, because, well, that's what
anonymous means! The in-class question solutions are working fine, did
you check to see if the answer appeared at the *bottom*? (The problem
part is repeated identically, I just add an answer in small print at
the bottom of the page) Also note that you can't see the questions or
answers if you're coming in from an off-campus computer link, like
AOL.
HW schedule question
I think I'm alittle confused as to how many capa's we have left? The
next one we pick up # 14 , when is it due?
This set is #14, it's already in the bins, and it's due the *Tuesday*
after T-giving break (Dec 1). There will be one more, issued Monday
Nov 30 (i.e. right after you get back from T-giving break) and due the
last Tuesday of the semester, (Dec. 8) i.e. 8 days later. That'll be
it, 15 CAPA sets for the semester!
CAPA 14 questions
>on capa 14 and question number 5: In your hint you say that
>you can see the max extension. Then you say that you can see the
>angular frequency? I am not sure what you mean, or how to see the
>angular frequency.
We had this on the board (on the left side) before a couple of
lectures, it's also Giancoli Eq 11-8a: the formula for x(t) in SHM is
x = A cos(omega*t).
So, you can just read off A (the number outside the cosine) and omega
(the number inside the cosine, next to t) Remember that omega = 2*pi*f,
and you already have formulas relating frequency f to k and m...
Final Week
Studying for final
>It would be of great help if you could list the exact section numbers
>in the book that we are responsible. We skipped around so much that I
>just want to make sure that I got all of the correct sections down in
>my notes. It w ould be very helpful!!!
You are of course still responsible for all old material (up to Exam
III, which went up through 9.5) that you already new about for earlier
exams. For the *new* material (since exam III), here are the sections
you are responsible for. Basically, anything covered in lecture, lab,
or CAPA is fair game.
9.6
10.1-6
11.1-4, 6-9,11,12
12.1,2, 5,7
13.1-4,
14.1,2,4-6
Shuttle tile question
>Dear Professor Pollock,
>
>I saw "Armageddon" the other night and in the movie there is a space
>shuttle going to an asteriod and the outer layer of the shuttle is
>stated to be made of titanium alloy. How would this compare to the
>shuttle tiles you showed us in class?
Haven't seen the movie, but it wouldn't surprise me if they were sloppy
with their science facts, since most Hollywood movies are painfully
incorrect in their use of physics!
The shuttle *tile* which I showed the class is glued on to the outside
of the shuttle. It's made almost entirely of silica (basically, a type
of glass) According to NASA's shuttle website, the majority of the
sections of the shuttle body itself are constructed of conventional
aluminum and protected by "reusable surface insulation". That's NASA
speak for the tiles, although I believe that they may be modifying the
tiles to more modern (more flexible, in particular) materials.
Cheers,
Question about HW 15
>I have a simple question about #6 on the Capa. Acutally it is not
>really a qquestion but rather confusion. I just am having a hard time
>seeing how to get the answer. Can your give me a hint or some helpful
>word of knowledge that can get me pointing in the right direction. The
>only hint on the Capa is to remember both hands. I remember that but
>can figure out the rest of the problem. If you could help that would
>be great. Thanks
I added some more to the CAPA hint on this one. This is a work -> heat
problem. When you rub your hands, you do work. Work = force *
distance, so you can figure out work done each *second* since you know
the distance the hands move each second, and you can easily figure out
the force of friction. This work presumably all goes into heating your
hands. So you should be able to figure out how much your hands heat up
*each* second....
Hope this gets you started
Questions about the final
>I just have a question or two about the final. Is it going to be
>cummulative in the fashion that the other exams have been cummulative
>(ie will the bulk of the exam be focused on what we've covered since
>the last test?), or will it all be covered equally? Also, what is the
>best way to study for your final? (Going over old tests, spend more
>time on new material, etc?) Any insight you could give me would be most
>helpful.
The final will be approximately 2/3 focussed on *new* material since
Exam III, and 1/3 review of older stuff. I would guess that the best
way to study would be just how you've studied for previous exams (in
particular, you should devote 2/3 of your effort to the "new" material
- fluids, SHM, waves, and heat) Reviewing old exams would be good,
also working out old CAPA's, trying some odd-numbered Giancoli
problems, maybe doing some of his web-site problems (e.g. "MCAT" type
problems) Whatever works best for you...
Some questions from the heat and thermo material
>Hi! I'm having trouble with the mixing water and ice problem. I followed
>and understood the similar example problem in Giancoli. However, when I
>used the similar method to calculate the equilibrium temperature of the
>CAPA problem, I obtained a negative temperature. That does not make
>sense; it is expected the eq. temp. should be between the two initial
>given temperatures (0 and 91 degC for my set). What am I doing wrong? (I
>checked that all the units and algebra were indeed correct).
You are right that equilibrium temp has to be between the two initial
ones. I'm not 100% sure which problem you're referring to. If it's #4,
there's a sort of a trick - if you pour water onto ice, you might not
melt all the ice! If that's the case, the water you pour in will cool
down to 0C, and some ice will melt (but not change it's temperature, it
stays 0C). You have to check for this special case, and I think that's
what's going on in this problem. Let me know if this doesn't help you.
>Also, if thermal expansion expands all sections, why does expansion not
>occur in all directions (to fill a hole cut from an object)?
Did you read Giancoli's conceptual explanation of why holes expand,
rather than filling in? Think about what a thin RING does when you heat
it. All the parts of the ring expand, expanding the ring, making the
hole BIGGER. A hole in a metal plate is very much the same thing. Keep
thinking about it till it makes sense to you!
>Is water the only exception to the rule that objects expand when heated?
First of all, water is *only* an exception to this rule in a very
narrow range of temperatures quite close to 0C. (0-4, I believe)
Secondly, it's not the only exception. E.g. if you heat a rubber band,
it will tend to shrink.
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