The Sum of the Forces

Hello everyone-

Since I've received a lot of inquiries regarding final grades, I just thought I'd make a few comments here- Hopefully these comments are still in some of your RSS feeds and you'll see these comments.

I am not going to post the exact cutoffs between A/B/C/D/F- suffice it to say that they are lower than the 90/80/70... cutoffs I posted on the syllabus (that's to your advantage). You have enough information to determine your final score given the information in e-gradebook.

The average grade (~74) and the median grade (~75) fell squarely in the 'C' range, where they should.

I try very hard to be absolutely fair about how grades are determined- with the exception of determining the cutoffs (over which I retain a small amount of discretion), determination of grades is entirely objective and follows the weighting scheme laid out in the syllabus. Indeed somebody in the class got the highest B+ and missed an A- by a fraction of a point; at the same time somebody got the lowest A- and missed getting a B+ by a fraction of a point. This is one of the unavoidable consequences of discretized grading schemes.

jay-

Posted by Jay Austin at 10:12 AM | Permalink | Comments (0)

Unless the university is officially closed on Monday morning (there is no indication so far that this will happen) the final will run from 10-11:55 Monday morning as scheduled. There is another final in the room after us, so if you want the full two hours to work, please make accommodations to get to the final on time!

jay-

Posted by Jay Austin at 7:55 AM | Permalink | Comments (0)

Just a reminder-

please get any outstanding lab reports to your TAs by Monday AT THE LATEST. I'd like to be able to get the grades posted for the class ASAP.

jay-

Posted by Jay Austin at 10:55 AM | Permalink | Comments (0)

Hello everyone-

I've posted a key to the hot air balloon problem we did yesterday, as well as to the practice exam we worked on today.

I managed to pretty badly mangle things on the second problem of the practice exam today, as several of you pointed out. I over-specified information in the thermodynamic cycle problem, so that the problem was internally inconsistent. I've revised the problem and written up a solution. I rushed preparing the problem this afternoon before class and I didn't think it through thoroughly. I apologize for the oversight and hope that the explanation in the key will assure that you come out of the practice exam experience more, rather than less, prepared for the exam.

With regards to studying: The cumulative multiple choice part of the exam covers kinematics and linear dynamics (no rotational dynamics or statics). It really focuses on your conceptual understanding of concepts like velocity, acceleration, force, momentum, energy, and the relationships between these. It doe not call for a calculator.

The rest of the exam is a bit more traditional. It will cover fluids, temperature and heat (and the relationship between them), the First Law of thermo and ideal gases. One of the problems asks for an explanation rather than a calculation; the rest are fairly straightforward (and well-posed) calculation problems.

See you Monday, 10:00, in MWAH 195!

jay-

Posted by Jay Austin at 4:35 PM | Permalink | Comments (0)

I've posted the key to HW 14 online. For a change, I've posted no new homework. Whew.

If people have suggestions of specific problems or types of problems they'd like to see covered tomorrow, please let me know ASAP so I can work something up.

The final exam will consist of two parts:
1. A 26-question multiple choice test, similar but somewhat more quantitative than the diagnostic you took earlier. This focuses heavily on the basic concepts of velocity, acceleration, force, energy, and momentum.
2. Problem solving, covering fluids at rest (hydrostatics/buoyancy/etc), heat and temperature (specific heat/heat content and heat transfer), the first law of thermo, and ideal gas law stuff.

jay-

Posted by Jay Austin at 3:10 PM | Permalink | Comments (0)

Apparently I didn't change the due date on Homework #14. It is due tomorrow at the beginning of class.

jay-

Posted by Jay Austin at 4:36 PM | Permalink | Comments (0)

The 14th and last homework for the class is posted now. See you Monday!

jay-

Posted by Jay Austin at 4:56 PM | Permalink | Comments (0)

The syllabus incorrectly states the time of the Final Exam. The final exam for PHYS2011, Fall 2008, is 10:00-11:55 on Monday, 15 December. We will meet in MWAH 195, as usual.

jay-

Posted by Jay Austin at 1:52 PM | Permalink | Comments (0)

There was an error on the key for the last homework- unfortunately, the grader did not pick up on it.

on 14.6, the book you have states that the area of a lid is 77m2. In the copy of the text I have at home (must be a different version) it states it as 77cm2, which makes a lot more sense for a lid of negligible weight. As you might imagine, this makes a great deal of difference to the answer. Since I'm guessing the grader took a point off of everyone's paper for an incorrect answer, I'm going to put one point back on everyone's HW12 (who turned it in), to save the trouble of digging through all of the assignments again.

I will post an addendum to the key.

jay-

Posted by Jay Austin at 9:10 AM | Permalink | Comments (0)

On the written out problem on homework 13, I am only concerned with the amount the pond would heat up from the sun shining on it. Ignore all other heat transfer mechanisms.

In the real world, there are other mechanisms at work as well, of course.

jay-

Posted by Jay Austin at 8:33 PM | Permalink | Comments (0)

Hello everyone-

I don't like the order the book presents the material in this chapter, so I'd like to discuss heat transfer mechanisms (18.12) today, and postpone the first law of thermo (18.9-18.11) until Wednesday.

Therefore, I'd like to postpone HW problem 18.46 until the next homework. In it's place, please solve 18.57. If you've already done 46, hold onto it and turn it in with HW 14.

jay-

Posted by Jay Austin at 11:07 AM | Permalink | Comments (0)

I have a misprint on the homework due Wednesday- in the first problem, I define apparent weight to be the true weight of the object plus the buoyancy force- it should be MINUS the buoyancy force. Weight is a force fdirected downward. You should find that the concrete blocks have a positive apparent weight (just like any of us have a positive apparent weight standing around in air) and the steel sphere float will have a negative apparent weight.

jay-

Posted by Jay Austin at 4:01 PM | Permalink | Comments (0)

The practice exam (with some annotations) is posted on the website. Also, a cheat-sheet for tomorrow's exam.

jay-

Posted by Jay Austin at 3:23 PM | Permalink | Comments (0)

Homework 12 is posted, as is a key to homework 11.

I've recently seen three videos which all look like great exam questions. Take a look at these, and think about the physics (or lack thereof) behind these:

http://failblog.org/2008/11/19/shopping-cart-fail/

http://failblog.org/2008/11/18/logistics-fail/

http://failblog.org/2008/11/17/cable-fail/

jay-

Posted by Jay Austin at 11:48 AM | Permalink | Comments (0)

Hello everyone-

Homework 11, and the key to homework 10, are now posted on the website.

In answer to several people's questions: yes, there's an exam next friday. It will cover rotation and statics.
We'll discuss it tomorrow and hopefully we'll be able to fit in a practice exam.

jay-

Posted by Jay Austin at 1:01 PM | Permalink | Comments (0)

Hello everyone-

I'll start lecture on Wednesday by discussing problem 7, which is very similar to the ruler problem we did in class on Monday.

jay-

Posted by Jay Austin at 8:11 AM | Permalink | Comments (0)

Hello everyone--

I just got homework 10 posted.

A few people have asked that I go over cross-products, so we'll do a short review of the idea of a cross-product on Monday.

We will be starting on Statics on Monday- we're only a day behind the reading schedule right now. Make sure you've read 10.1-4.

We'll also go over problem 11.60 on Monday, and I'd like to add the following question to it: How much energy is there in the system before and after the student walks to the center? Is the amount of energy in the system the same or different, and why?

jay-

Posted by Jay Austin at 4:09 PM | Permalink | Comments (0)

Hello everyone-

The key for HW 9 is posted; I'll post HW 10 problems first thing tomorrow.

Tomorrow, I am going to start the lecture by discussing the following problem:

You fire a bullet, of mass 9.00g, at 860m/s into a wooden door of mass 18kg and width 0.85m. The bullet hits the door at a right angle, 0.75cm away from the hinges (the axis of rotation), and becomes lodged in the wood. The hinges are frictionless, of course. What is the resulting angular velocity of the door?

Posted by Jay Austin at 4:13 PM | Permalink | Comments (0)

Hello everyone-

I didn't get through as much stuff yesterday as I would have liked. I'd like to postpone homework 9 until Thursday so that we have a chance to talk about a few more concepts tomorrow before you have to turn it in.

jay-

Posted by Jay Austin at 3:05 PM | Permalink | Comments (0)

Hello everyone-

I hope the practice exam session was helpful for you. I've posted the exam and an abbreviated key on the website.

jay-

Posted by Jay Austin at 4:22 PM | Permalink | Comments (0)

Homework 9 is posted. It's a bit shorter than recent homeworks.

jay-

Posted by Jay Austin at 1:37 PM | Permalink | Comments (0)

A key for HW #8 is posted, as well as the cheat sheet for exam 2.

Tomorrow, we'll have a practice exam, which will be some sort of interactive session where you'll get a chance to work on a problem/ problems similar to those you might find on the test. I'll post a copy of the practice exam tomorrow after class.

jay-

Posted by Jay Austin at 3:53 PM | Permalink | Comments (0)

Hello everyone-

I've posted a cheatsheet for exam #2 on the website.

jay-

Posted by Jay Austin at 11:21 AM | Permalink | Comments (0)

Hello everyone-

Here's the schedule for the next few days:

Monday 27 October Chapter 10.1-4 (Rotational Kinematics)
Wednesday 29 Oct Chapter 10.5-10 (Moment of Inertia) (Homework 8 due, Key released)
Thursday 30 Oct Review of chapters 5-9
Friday 31 October Exam II, covering chapters 5-9.

I appear to be about a day behind where I'd planned on being.

For Monday, a good problem to get you on track will be 10.6.

jay-

Posted by Jay Austin at 4:14 PM | Permalink | Comments (0)

I've fixed the link for homework 8. Glad to see people aren't waiting until Tuesday to download it...

jay-

Posted by Jay Austin at 9:05 AM | Permalink | Comments (0)

Hello all-

In order to give people a chance to finish an assignment on momentum, hand it in, and see a key before taking the exam, I am postponing Exam II until 31 October. Homework 8 will be due on 29 October (Wednesday) as originally scheduled.

One day next week, we'll do a review and a mini-practice exam.

For tomorrow, I'll start with a discussion of 9.56, an inelastic collision problem.

jay-

Posted by Jay Austin at 4:20 PM | Permalink | Comments (0)

Hello everyone-

I have just filled out the paperwork on mid-term alerts- they will be sent out tonight. Just so you know, I took your average homework grade and your exam grade (including the 'supplemental' exam) and averaged them- if the result was 65 or less, I sent one out. You should have enough information available to you from e-gradebook to determine why you got one (or didn't). Performance in the lab is not included in this score.

I am not convinced that one exam and a few homework grades are a reliable predictor of your ultimate performance in this class, but it is all I have to go on.

While I do not actively encourage people to drop the course, I will remind you that the final drop date is 27 October. You should individually assess your chances of passing the course and act accordingly. I am happy to discuss this with people on a case-by-case basis in my office.

jay-

Posted by Jay Austin at 11:05 AM | Permalink | Comments (0)

With regards to the extra credit assignment, many cardio machines have outputs that tell you about your power exertion. the two most common units are Watts and Calories per hour (which is an energy unit per time unit, or power).

Here's the trick: Machines that give an output in watts typically are referring to the actual mechanical output you're exerting, estimated using sensors embedded in the instrument. The Cal/hr measurement refers to how many food calories you need to be burning in your body to produce your rate of output. The human body, as with anything that converts one form of energy to another (chemical potential to Mechanical, in this case), is not perfectly efficient; in fact, you need to burn 4-5 calories of energy for every 1 calorie of actual mechanical work that you do.

This means that if you are using calories per hour to estimate your power, you need to divide this number by about 4 to get your actual mechanical energy output. For instance, for a rate of 800 calories per hour:

800 chem. Cal........1 mech. cal........ 4184J..........1 hr.......1 W
---------------------x- ---------------x---------------- x---------x -------- = 230 W
1 hr.....................4 Chem. cal........1 calorie.....3600s....1 J/s

Without the factor of four, your mechanical output will appear to be artificially high when you convert it to watts.

If you don't get this message in time to make adjustments, don't worry about this. I didn't realize that was what exercise equipment measured until somebody just brought it up. Just don't be too surprised if you used a cal/hr reading and your numbers are wildly high.

jay-

Posted by Jay Austin at 11:09 AM | Permalink | Comments (0)

Taking a closer look at some of the problems for Homework 7- Problem 8.17C will now be extra credit. (You still need to do parts A and B, and answer part (D) for (B)).

jay-

Posted by Jay Austin at 10:57 AM | Permalink | Comments (0)

On Monday, we're going to start talking about Momentum and related topics. Please read sections 9.1-9.4.

We will discuss Problem 9.3 in class as well.

Another good practice problem from 8 is 8.53.

Posted by Jay Austin at 3:08 PM | Permalink | Comments (0)

Okay, I found the sign error. We were looking at #8.59. I noted that there is a change in the mechanical energy of the system, due to energy being lost to friction. The mechanical energy is initially all kinetic (K1) and at the end state is all spring potential (U2).

So we know that

Wf=Emec2-Emec1=U2-K1

And the problem asks for the initial velocity of the mass, which I can figure out given the Initial kinetic energy:

K1=U2-Wf

We determined Wf to be -0.46J (friction does negative work). Where I got flummoxed was that I had dropped in -0.9J for the spring potential energy, whereas that is actually the work done by the spring- the spring potential HAS to be positive: +0.9J. Drat.

Anyway, the initial KE then is

K1=U2-Wf=(+0.9J) - (-0.46J) = 1.35J

From which I can calculate the initial velocity: 1.0m/s.

Posted by Jay Austin at 2:56 PM | Permalink | Comments (0)

The key to HW 6 is posted now, as is Homework 7. I've included a (hopefully fun) small extra credit problem as well.

Posted by Jay Austin at 8:51 PM | Permalink | Comments (0)

We'll start tomorrow with a discussion of problem #22 in chapter 8.

jay-

Posted by Jay Austin at 3:11 PM | Permalink | Comments (0)

Hi Everyone-

Since homework 7 will have just been turned in, I think I'd like to delay exam II for just a few days. My current plan is:

* Homework 7 key distributed 23 or 24 October
* Homework 7 returned on Monday, 27 October
* Exam II delayed until Wednesday, 29 October
* Homework 8 due date delayed until 30 October.

If you have a conflict with the new exam date, let me know immediately. Exam 2 will cover everything so far through chapter 9.

Just for fun, I've posted a short writeup off my experimenting with the rowing machine. It can be found in the list of documents to the right, or click :
here

Posted by Jay Austin at 4:30 PM | Permalink | Comments (0)

I'll start the lecture on Monday with a discussion of problem 65 from Chapter 7.

Posted by Jay Austin at 8:36 AM | Permalink | Comments (0)

Hello everyone-

Homework 6 is posted.

I know this is late notice, but I'll start class today with problem 54 from Chapter 7.

jay-

Posted by Jay Austin at 10:30 AM | Permalink | Comments (0)

Hello all-

I've posted a short writeup on how/why vector components come out the way they do in the sloping ramp problem.

http://www.d.umn.edu/~jaustin/PHYS2011_F2008/vector%20components.pdf

Posted by Jay Austin at 5:27 PM | Permalink | Comments (0)

I've been getting a lot of questions about e-Gradebook recently- I view e-Gradebook as simply and strictly a method to communicate your homework and exam scores to you, as I have them recorded- I do not have e-gradebook do any calculations, mostly because of the lack of flexibility it shows in the face of oddball grading schemes (i.e. dropping low homeworks, diagnostic exams that count as free homeworks, extra credit on exams, etc).

You have enough information, between these scores and the grading scheme set out in the syllabus, to evaluate your own progress.

jay-

Posted by Jay Austin at 8:29 AM | Permalink | Comments (0)

Lab Schedule:http://umd-physics.d.umn.edu/physlabs/labsched.htm

Help Session Schedule:
http://umd-physics.d.umn.edu/physlabs/helpsess.htm

Posted by Jay Austin at 12:29 PM | Permalink | Comments (0)

I've posted the scores for the extra credit portion of exam 1; add this to your original score and that's your exam 1 grade, out of 100.

They seemed to go much better than the equivalent problem on the exam. I hope this gave some people a chance to solidify their understanding of 2D kinematics.

The class average was about 14 out of 24; this sounds low, but I graded these fairly strictly, unlike the exam which was graded fairly easily. This brings the class average up to about 75, which is where it should be.

jay-

Posted by Jay Austin at 9:46 AM | Permalink | Comments (0)

Hello all:

I've posted a key to homework #4 ( I didn't get to finish 5.8, and I have to leave; hopefully I can get back to that next week). I've also posted homework #5, which is focused on forces.

Next monday, We'll start the discussion with 6.38, and we'll spend most of the day talking about problems out of chapters 5 and 6. If you have specific problems you'd like to see addressed, let me know in time that I can prepare something.

Starting Wednesday, we'll be into chapter 7. Make sure you've done the reading by then.

Have a great weekend!

Posted by Jay Austin at 4:42 PM | Permalink | Comments (0)

There are help sessions run by grad students available, that I just found out about. The schedule can be found at:

http://umd-physics.d.umn.edu/physlabs/helpsess.htm

jay-

Posted by Jay Austin at 4:52 PM | Permalink | Comments (0)

I'll start tomorrow's lecture with a discussion of problem 23 from chapter 6.

If somebody lost a calculator in class today (Thursday) let me know.

Posted by Jay Austin at 4:50 PM | Permalink | Comments (0)

Tomorrow, I will start the lecture by going through problem 32 in chapter 5. You're not required to do it but you will find the discussion of the problem much more useful if you do!

jay-

Posted by Jay Austin at 9:26 PM | Permalink | Comments (0)

I neglected to put point values on the exam problems:

1A-D: 10, 10, 5, 5
2A-D: 10, 10, 5, 5
3: 20
4A, B: 10, 10

Posted by Jay Austin at 9:25 PM | Permalink | Comments (0)

Back from Madison. Hope the lecture on Monday went well. I'm getting a lot of panicky e-mails regarding the results on the first exam.

First thing: don't panic. Deep breath.

Second: I've posted an exam key on the blog site. I strongly, STRONGLY suggest you go over this key very carefully, try to understand how I approached the problems.

Third: For those of you wondering what to do: One thing is to please take advantage of office hours! A very small subset of you it taking advantage of a chance to discuss this stuff one-on-one with me.

I am considering a way to pull up the class average- My goal is to provide some mechanism to boost the exam average by about 20 points. Which is not to say that I'm going to just add 20 points to everyone's score; rather I am considering ways for you to earn said 20 points. More on this later as I work out details.

Finally, I would like to postpone the due date for homework #4 until Thursday, so that I have a chance to talk about the sorts of problems on the homework a bit more in class.

I will be in my office most of the day tomorrow if you would like to drop by and discuss any of this.

Posted by Jay Austin at 3:33 PM | Permalink | Comments (0)

Hello everyone-

The scores from the first exam have been posted. The exam average was about 62%- significantly lower than I would have liked. I will consider a mechanism for providing people an opportunity to boost their score.

I've given similar exams in the past and have had significantly higher means- Apparently the minor changes I made to the exam made more of a difference to the difficulty of the exam than I anticipated.

I will discuss this briefly on Wednesday.

Posted by Jay Austin at 9:11 AM | Permalink | Comments (0)

Hello all-

I've started grading the exams- hopefully I'll be able to finish this weekend and get grades posted by tuesday at the latest. I will hand them back on Wednesday, most likely. I'll try to post a key as well.

Another professor will be filling in for me on Monday. I am going to be down at UW-Madison giving a lecture.

Have a great weekend!

jay-

Posted by Jay Austin at 4:45 PM | Permalink | Comments (0)

Homework 4 is posted now. Problem 1 and the book problems from ch4 will be good exam practice, so you may as well get them done now.

I'll be in my campus office tomorrow hopefully from noon on.

Posted by Jay Austin at 4:39 PM | Permalink | Comments (0)

Hello everyone-

Homework: The key to HW #3 is posted. Please use these keys as a resource- they're fairly exhaustive solutions to the homework problems.

Exam: the exam is Friday, covers chapter 1-4. I'll provide an equation sheet identical to the one posted here. There will almost undoubtedly be a 1D motion problem, a 2D motion problem, something about non-constant acceleration, maybe some drawing/sketching stuff.

Lecture: Again, I can's strongly enough recommend tonight's talk at 7:00 in Weber Music Hall: "The Great Lakes Water Wars". It's really interesting stuff.

Posted by Jay Austin at 3:14 PM | Permalink | Comments (0)