Over, and more!

This course is officially over now!? It feels very abrupt to me, certainly, especially after all the work-load that kept me on the verge of nervous breakdown for the past one week or two, right up until today... I guess I now know a thing or two about teaching a large course like this!!

Thanks to all of you who participated in all the discussions. Many of those discussions were great!

It was a great pleasure to be your instructor for this course! Sorry to have made many of you feel "uncomfortable" in exams and such. Although I did not really mean to do that, I realize that it is something necessary to some extent.

As for the final outcome, that letter grade, I believe you can check it out now, since I posted all of them today.

Also, the solutions to the final exam are uploaded in the sols+ folder.

I have many thoughts about this course. Many things were enjoyable and some weren't. I am sure most of those thoughts will be good for me in the future. If you have some helpful thoughts to share with me, come knock on my office door in the new year.

I hope all of you have happy holiday breaks and come back fresh for more constructive struggles!

In the mean time, when you look up the sky with your loved one(s) at day or night during this holiday season, chew on this question, if you don't have any other thing to think about (or even if!). We had this thing about the circular orbit of a space shuttle. Without doing anything else than just applying very quick forward thrusts it is possible to put this shuttle in another circular orbit with a larger radius. The surprise is that the speed of the shuttle is smaller in the larger circular orbit! Just one thrust will not do, but two exactly measured, very quick, thrusts will do! It may sound a little absurd. How is it possible that each time the thrust is applied the speed increases, but, in the end, the final speed is less than the initial speed? Actually, this is exactly what happens! [Partial answer: the intermediate orbit is not a circle!]

Happy holidays!

The subtle thing

OK, the center of mass moves like a point mass (= total mass of a compound object) under the influence of external forces only.

Now consider a non-center-of-mass point of the compound object. Does the external force have any influence at all on that point? The answer is Yes, Yes, Yes! In general, any point of an object is under the influence of external and internal forces. The center of mass happens to be where all internal forces magically cancel out! It does not mean that external forces do NOT apply to other points.

However, since it often suffices, or it is the most important, to figure out the motion of the center of mass, we are used to reducing a problem involving a complicated compound object to a point mass problem (without forgetting an internal rotation or an internal motion, if applicable), but we should never forget that external forces act on other parts, and the center of mass feels external forces through those other parts around it.

Just a subtle point to make clear, in order to avoid any misunderstanding. Maybe it is already clear to many students?!

Done with homework!

I've downloaded all homework scores and, even though the MP program might be still recording scores, those scores won't be reflected in my grade book any more.

So, we are done with the homework!!!
Kudos to all of you for all the hard work!!!

All the solutions are available at the "sols+" folder at the course web site.
I also plan to upload solutions to the final practice exam "soon"!

Review session schedules

Some students still ask me the question, "is the final going to be cumulative?".  The answer is No, and then Yes.  It is No, since it will cover rotational motions and afterwards, as I announced it before.  However, it is Yes, in the sense that if you do not have cumulative knowledge of what went on before, it would be very hard to do problems!  Practically speaking you should review Homeworks 6-9, PracExam, Bear problem, your note, lecture notes, book.

Fri Dec 5 2008

3:30PM - 5:00PM

Space Assignment(s): Nat Sci Annex 101

(space is limited; only ~ 100 seats;

if you are unable to attend the sunday session, please come early to this one!)

Sun Dec 7 2008

3:00PM to 4:30PM

Space Assignment(s): Thimann 003

That "just for fun" problem

I think the discussions and questions for that just for fun problem of going through the earth were very good.

Some more thoughts here.

(1) If the density of the earth is dependent on radius, becoming larger as radius becomes small, how would it affect the round-trip time?  Ans: It will shorten the time.

(2) In this case, will the shell theorem still applicable?  Ans: Yes.  See my lecture note about the exact statement of the shell theorem.  Newton's shell theorem is applicable when the density is the function of r only.   The density does not need to be uniform.  As long as the density is not orientation dependent (again an assumption here), the shell theorem is valid.

(3) In all these "calculations," simple and "stupid" assumptions like uniform density is made.  Why should I trust the calculation?  Ans: In engineering, the inaccuracy of these results may be viewed as great deficiency.  In physics, the inaccuracy of these results may be viewed as problematic in some cases (if you are trying to build a high precision experimental equipment based on these estimates) but it would not prevent the results from being interpreted as insightful in other cases (if you are after only an order of magnitude estimate, to be compared with other numbers based on other views).  Generally, these order of magnitude estimates are very valuable, and so "stupid" does not really mean that.  Even if you are into doing fancy computerized calculations, these simple calculations give you an idea what the answer should be approximately, and play a great role of preventing mistakes in super duper computer calculations.  Physicists like to simplify things and draw a rough sketch first.  More often than not the sketch already contains all essential physics! 

When I say "stupid" or "crazy" in class, I actually likely to mean "smart" or "cool"!

(4) Just to make sure people are on board on this.  To avoid Coriolis force to mess up with our "travel through the center of mass" (since the earth is spinning), we need to do it from the north pole to the south pole, or the other way around.

Those were great questions!

Comments about the EC (format)?

The Doodle polls about the EC exam are coming in. Nice.

If you like to let me know your reasons why you voted so in the Doodle poll, please do leave comments here or just send me emails.

Update: The overall poll results so far (Dec 05, 2008) are in.

Poll: Was the EC better LEARNING experience?

Yes, it was better learning experience.	No, I did not learn much.	Don't know -- probably no difference.
53	34	27

Poll: Was the EC better experience for being EVALUATED?

Yes, I felt better about being evaluated this way.	No, I think students can't get evaluated fairly using this type of exam.	Don't know -- probably no difference.
29	52	27

All these votes, except couple votes, were collected before my actual evaluation went out. I do not know whether there is any difference between "before" and "after."

Thanks to those who voted!