QR: The force that gives back...

It appears that not all students were able to hear the good explanation that was given by the student in class on today's (only) quiz question. Sorry about that. So, let us have one or two students explain the answer to this question here on-line. Please volunteer and leave comments (especially if you need some extra credits).

LN: Potential function is well-defined for a conservative force

Up to a constant, that is.

We went over this in today's lecture, but I re-summarize it below in a slightly different way. I think it is better. In particular, the substitution of dr with -dr, as done in class, was not necessary, and that step was not particular clear in the lecture, I think.

HW: Set #5

6.62: You need to look up the last table ("energy contents of fuels") of Appendix C of the textbook.
6.65: Review Example 6.9 first.
7.21,7.24: Review Examples 7.4,7.5 first.

Good luck!
Questions can be posted as comments here.

Someone suggested that I give the answers to even numbered questions of the book that I use for homework (with numbers modified by a random number generator). I think it is a fair request, since then you can practice from the book to see if you get the answer right, before working on the actual MP question. So, here are the answers to the selected textbook questions (not the MP questions):
6.16: 1.9E8 N
6.80: 870
6.69: 42 kJ
7.16: 7.0 MJ and 1.0 MJ
7.18: 5.2 J
7.24: 2.3 kN/m

HW: Set #4, RPM and Chandelier

Perhaps this is a cultural thing. Vinyl record listeners probably knew "RPM" better.
RPM = revolutions per minute
If you express the period T, in minutes, then that is "minutes per revolution," i.e. the inverse of RPM.
Hope that helps.

It appears that the "Chandelier" extra credit problem causes lots of grief, due to the computer formatting problem. I tried to give all due credits back when students email me explaining [calmly, in an ideal world] how dumb the computer program was not to recognize their correct answer. Fair enough. Did I miss giving you the credit that you deserve? Did you have a similar problem that you did not email me about? In either case, let me know by email, right away!

You can ask for your due credit for any problem, when you are sure that your answer was correct. For instance, if you can verify that your answer is just another form of expressing the answer that the computer gives you, then you know that your answer must be correct. Also, please note that parenthesis and other formatting matters get in the way of computer understanding your answer.

When you do send me an email to get your due credit back, you should say that explicitly. That is, don't just say what headaches that you are getting from the MP program, but do say that you want your credit back if that is what you want.

HW: Beware of Red Herrings

Please know that some problems have too much information. If you use the information that is unnecessary for solving the problem, you may find that you are required to know strange things... Typically, if there is something that we really did not discuss in class, then it is likely to be a red herring.

In homework #4, the "rolling friction and bicycle tires" problem, is the pressure information a red-herring? Why don't you try solving the problem without using the pressure information first?

An example of a red herring in the past was the terminal velocity of a sky jumper. That terminal velocity was not essential for figuring out the answer.

A red herring does not necessarily mean a bad taste, in my opinion.

One more note: The rolling friction is similar to the static friction in its origin, but is much weaker. For the purpose of the above problem, just treat it as "another friction," like we did in class for static or skidding (thus kinetic) friction of automobile tires.

HW: Hints for 5.70

This problem is a "mechanical advantage" problem. Here is a hint question to help you, if you are unsure about this problem. How many ropes are pulling the load, effectively? In other words, if the tension of the rope that the worker is pulling is T, what is the effective force that the rope is applying to the pulley? If you do not know the answer to this one for sure, consider the free body diagram for the pulley. You may ignore the mass of the rope (implicit assumption here) and may draw a small rectangle that tightly fits the pulley. If you do so, you may consider all things included in that rectangle as a single compound object. Or, even more simply, you may draw a bigger rectangle that encloses the pulley and the load, and consider all objects inside that rectangle as a single compound object as heavy as the load (again, assuming other things do not weigh any at all).

HW: Hints and Grading in MP

The grading scheme in the case when hints are involved can be confusing. The grading policy says you "get credit for correctly answering a question in a Hint," but it also says you lose credit if you exhaust your answers or request the answer to the Hint. But, how does this work exactly?

Here are some texts from the MP help file, for you to read through and understand, in case this is still a mystery. Some points worth knowing are: (1) if you do not open hints at all you get 2 % bonus, (2) if you open hints but simply read them without submitting any answers, there is no penalty and no bonus (as long as your answer to the main part is good, I think), (3) if you open a hint and submit a solution to that hint, then that hint becomes part of the problem, weighed equally as any other part, (4) if you request an answer to, or exhaust the allowed number of tries for, a hint or the main problem part, then you lose points.

I hope this gives you some ideas, while it may seem (to me also) at times that it is impossible to understand the scoring system.

MP help texts below this line.

--------------------------------------------------------------------------------------------

For example, suppose a problem part has:

▪	3 question hints, each with 1 question to answer

▪	1 final answer

Case 1:

Student answers all hint questions correctly and also gets the final answer. Each hint question counts for 1/4 of the possible grade. Student has answered 3 hints and the final answer correctly, for a total of 4 out of 4. Student gets 4/4, or full credit.

Hint 1	100%
Hint 2	100%
Hint 3	100%
Main Part	100%
Part Score	100%

Case 2:

Student opens hint 1, answers it, understands how to work the problem, doesn't bother with the other 2 hints, goes right on and answers final question correctly. The student has correctly answered 1 hint and the final answer, and gets credit for having answered the other 2 hints correctly because the final answer is correct. 4/4, or full credit.

Hint 1	100%
Hint 2	100%	not answered; 100% because main part is correct
Hint 3	100%	not answered; 100% because main part is correct
Main Part	100%
Part Score	100%

Case 3:

Student opens hint 1, answers it correctly, goes directly on to final answer but gets that wrong (without exceeding the number of allowed answers). Student then goes back, opens other hints, and works them before succeeding in answering the part. Student eventually gets 4 out of 4 answers correct, for full credit.

Case 4:

Student answers all hints correctly but exceeds the number of allowed answers for the main part without answering it correctly. Student gets 3 out of 4 answers correct, for 3/4 credit. (This is a rare case; most students who answer all the hints correctly will get the final answer as well.)

Hint 1	100%
Hint 2	100%
Hint 3	100%
Main Part	0%
Part Score	75%

Case 5: Deferred credit

Student does not answer the main part correctly, does not exceed the number of allowed answers, but works 1 or more hints correctly. When the assignment is due, student does not get any credit because the main part has not been answered. (Remember: the main part must be answered.)

▪	However, if the professor gives partial credit for late work, the student can go back anytime until the end of the course and continue working for additional credit.

Case 6: The only case in which student is guaranteed to lose credit:

Student requests answer for a main part or a hint.

▪	If student requests answer to a hint: The student gets no credit for the hint even if the main part is correct.

▪	If student gets some hints right, but requests answer to the main part: The student gets credit for those hints, but not for the main part. The student gets no credit for any unanswered hints.

Student is not allowed to request the answer to the main part before opening at least one hint.

After the entire item has been completed, the student's total points are divided by the maximum points for the item equal to:

Max_points = number of main parts

This ratio is their score for the item, and can be greater than 1 if the student got mostly correct answers without using any hints and the hint bonus is on.

QR: The log and the friction

Hi, I like to re-visit this quiz question as well, since it appears that not everyone is 100 % comfortable with it.

Feel free to make comments.

QR: The Sun doing any work on the Earth?

Hi, this is "quiz revisited" for today's quiz. The question that I select is the last one, because I want to make sure that everyone understands the solution to this.
Please make comments about what the correct choice to this question is, and explain why. Also, change "the period of one year" to "one month," and then answer the same question.

NC: Scott Adams Quote (Dlibert)

Hi, my email mentioned something about the book "The Dilbert principle." The following quote is what I meant. This quote should totally lift you up (it definitely does me), not the other way around.

http://www.quotationspage.com/quote/32021.html

QR: Quiz Questions Revisited

Hi, class, as I receive late reports on the quizzes, I notice a pattern of some remaining misconceptions. Also, we simply do not have time to discuss everything in class. So, I suggest the following solution. I will upload Quiz questions as images here, so that you guys can revisit the quiz, at your own convenience.

I'd encourage you to leave your explanations to quiz questions in the form of comments to this message (and similar other future messages). I would like to see the simplest scientific explanation within the principles that we covered in class. Explanations based on everyday experience are OK, but only as far as the connections between them and the physics principles of our lectures are clearly made.

If a good final solution emerges from someone's comment, that person will be inducted into "the hall of fame of the best commentators," and she/he will be given the same bonus as a person who spoke out in class with a good explanation. As in class, I will also consider giving reasonable partial bonus points, for reasonable but incomplete comments. [Note: if you already got bonus points for the same quiz question or the same quiz session, please refrain from speaking out here.]

At this time, I have 5 quiz questions for discussions.

Here is Q1:

Here is Q2:

Here is Q3:

Here is Q4:

Here is Q5:

HW: Homework Grading Policy etc.

Some of you still ask me about homework grading policy and late homework policy. The answer is "check your homework." The following images should be self-explanatory. One thing that it does not say, though, is how late homework is accepted. That is one week, by which time you will get ~ 80 % of credit for the problem that you submit (as per the policy stated in the syllabus). Please note that the MasteringPhysics system is more lenient about late homework, than a traditional system, since it is per day and per problem basis. However, please do not make it a habit of submitting homework late, since it will be detrimental to your progress.

EX: That Tarzan problem

The solution to that Tarzan problem may be mathematically sound, but perhaps psychologically difficult to accept. In that case, try this "other solution," which tracks what goes on in Tarzan's mind as he pushes water and scans the shore line at each stroke.

LN: Weight and apparent weight ...

Weight can be a persistent, nagging, problem in physics, as we are finding in this class. Let me say a few words about it.

We defined weight to be mass times gravitational acceleration. One good (or bad?) thing about definitions is that we do not have to think about them!

Now, what is this concept about "apparent weight"? This is the weight that you actually measure. Right here is the confusion... Why do we not say "measure apparent wight" then? We could, and perhaps should, say that, but we usually don't. So, when questions ask "what is the measured weight" that means "apparent weight" not (the true) "weight"!! With this in mind, please review the last problem of Quiz_10-20 ("measured weight") and the first problem of Quiz_10-22 (weight vs. apparent weight).

Here is a related question for you. Suppose you are within a reference frame A (say an elevator accelerating down), which is accelerating relative to an inertial reference frame B (say your laboratory reference frame on a planet), in which the true weight is equal to the apparent weight. Let us say that unfortunately you do not know the gravitational acceleration value (taken to be constant; we are near the surface of the planet) in the frame B (i.e., you do not know which planet you are on!), nor do you know what the acceleration of frame A is relative to frame B. You have lived all your life in frame A. Can you ever figure out your true weight on that planet (i.e., in frame B), not the apparent weight in frame A? If so, how? If not, why?

Leave some comments, if you have brilliant/clever/mundane-but-sincere/just-so-darn-curious ideas, comments, or questions.

HW: New homework, new experiment

Hi, students and TAs of 6A, I created this blog to facilitate useful discussions pertaining to our on-going class. I have been receiving emails from students regarding homework problems or technical issues regarding the homework site. I've been thinking that an open forum like this may benefit more students.

I have never created any blog. So, this is a new experiment for me.

Please leave comments on this blog site if you have questions about homework or any topics worth discussing in relation to our class. Any students or TAs can leave comments and reply to other comments. Comments may be moderated. Any academic or technical topics related to class are welcome.

I would like very much to see interactions between students. If students interact within themselves and come up with solutions, that would be quite ideal and would benefit you the most in the long run. I will facilitate discussions, of course, and TAs are also welcomed to make comments as well.

OK, so much for the new experiment... The new homework (#4) is open for you. There are some extra credit problems. Good luck!