Last-minute things for the competition

Last minute preparations and information ....

Things: Tomorrow, I will bring the following items, at least:

• USB extension cable
  
• tape measure (verify ceiling height, etc)
  
• camera
• soldering iron (sensor repairs)
  
• stopwatch (elapsed time between shots)
  
• target

What to bring: The launcher (duh), position sensor, and the computer running your calculations. If you need a computer for tomorrow, you'd better let me know soon. Like right now.

Press: It is not clear if press will come or not - they won't know themselves until the last minute. They have been invited, so we'll see what happens. If they do show up, it would be nice if a few of you were a bit early to the competition so they can ask you a few questions. Obviously this is optional, but the basic point is, 'show up early, and there is a chance you'll be interviewed.'

Visitors: There are going to be a number of random visitors tomorrow. Pay them no mind, go on about your business :-)

Location: Shelby L107. This is not the same as 107, a lecture hall you may have had classes in.

Directions: from the rotunda entrance, go left. Continue down the hallway until you reach the breezway. Go through it and into the next hallway. At the dead end, turn right. L107 is on your right. I will have a few signs up to guide you.

Timing: We might be done by 12:30, it is hard to predict. We will not go beyond the normal class period.

Grading-related: Remember that you need to turn in your source for the trajectory calculations. Please try to do this by the end of Friday.

Ok ... that is all I can think of right now. The contest will be pretty relaxed, but we will have to be efficient - with 5 teams and multiple shots, it will take a while.

Physics Majors on Facebook

Join us.

Quiz 8 & 9 solutions

Well, answers anyway. Almost everyone aced quiz 9 (thermo). Quiz 8 was not quite so great ... but I think now the answers will be clear to you.

Moodle update coming shortly (give me like 30min) so you can see your quiz results.

Exam III solutions

Here you go. Let me know if you find any errors.

Physics Majors event

Just FYI, since a number of you are physics majors ...

---

All PHYSICS MAJORS!!!

Do you actually know other physics majors?? Yeah, that's what I thought. So come to the meet AND greet.

FREE FOOD! Yes, completely free. In fact, it will even be surrounded by nerds. So come.

DETAILS:
Who - YOU....hopefully...
What - Meet the other people who share your uniquely awesome interests
When - FRIDAY, as in THIS FRIDAY of dead week. which is May 1st, just fyi. 3:00-5:00pm
Where - Room 109 in Gallalee (obviously)
Why - because Tara and Ted were upset that we didn't know all of you

So, it'll be awesome, especially once you get there :)

-Your Fellow Physics Majors, Tara and Ted - and kind of the Society of Physics Students, kinda -

Old PH105 Exams

I put up some old PH105 exams for you to look at - some with answers only, some with solutions.

They are easy problems compared to what you've been doing, but good practice nonetheless. For the final it will be advantageous to be able to quickly solve a larger number of straightforward problems, as opposed to our usual exams where you have to solve only a few tough problems.

Tuesday's class

Today we'll go over some review questions, and spend an hour or so practicing for the rocket competition on Thursday.

Also, I had completely forgotten about Exam III solutions until one of you reminded me in the comments ... I'll get on that and have something for you by Thursday at the latest, along with solutions to the example problems from last week. The end of the sememster is proving a bit more painful than expected, so I am a bit behind ... apologies.

I have put up some very silly flyers to advertise our little contest. If you feel so inclined, you can do some of your own advertising as well. A number of faculty and staff members have expressed an interest in coming to watch the competition, mainly because they think what you've been up to is very neat. Probably, they are jealous.

(I may have mentioned this to the University Relations office. There might be local press.)

Finally, I'll post directions to L107 Shelby soon. You can find a handy map here. Note that you are going to L107, which is not the same as plain old 107. (Grab the 'emergency evacuation plan' map, it is actually much clearer. And possibly quite relevant, one never knows.)

Common Misconceptions

Nice link courtesy of Alex.

I liked this bit at the end:

"A telling comment on the issue of fairness in teaching elementary physics: Two students asked if I was going to continue asking them about things they had never studied in the class."

The answer is always this: HAHAHAHA yes.

(I am at least mostly kidding. There won't be any material on the final that we didn't cover in class, but there will be relatively simple situations that are new ... but situations which can be handled straightforwardly with what we covered in class. No surprises.)

Quiz tomorrow (Friday)

Reminder: there is a 5-question multiple-choice quiz on Ch. 18 tomorrow.

The content of the quiz will be restricted to the sample problems in Ch. 18 - if you can do those, you are good to go.

In addition we'll go over some practice problems for the final (the ones I put on the board today).

Today's class ... probably

I am mulling over a change of plans. On one hand, we should do more thermodynamics. On the other hand, you will all end up taking it again anyway ...

So here's my thought: if you agree to do a bit of reading on your own and take a short multiple-choice quiz on Ch. 18 on Friday, we'll spend tomorrow doing final exam review instead. Nothing we covered after Exam III is on the final exam, so this is a reasonable gamble for you to take, I think.

Anyway: tomorrow you have a choice - lecture on heat and thermodynamics, or do practice problems for the final. Since our PH125 Deathwatch is in its final stages, here's what the rest of our time looks like:

This week:
Thurs: practice or thermo + concept test + rocket testing
Fri: pending your vote for Thurs, possibly a thermo quiz. Post-mortem on the concept test (i.e., where are you as a whole having the most trouble). After thinking about Crito's comments & link, I'll also point out the ones I thought were tricky and why ... where does intuition seem to fail?

Dead Week:
Tues: final exam review, dry run for rocket competition
Thurs: rocket competition
Fri: no class

Thursday's class

Thursday, we'll be doing our requisite post-assessment. Remember that concept test you took at the beginning of the semester? Basically, we'll take it again and see if you learned anything :-)

It should take about 20min, and there will be grade-related incentives for doing well.

We'll also go over a bit more thermodynamics (heat and energy), and leave some time for you to do some 'practice shots' for the rocket competition.

Evaluations are today

If you were planning to stay home this morning ... we are doing course evaluations during the second half of class. FYI.

Last HW set

Here it is.

As I noted before, this one is optional - you don't have to do it, but if you do, it can replace your lowest non-dropped homework grade. Also useful for preparing for the final exam ...

Rocket launcher code

Linux kernel module to control the launchers. Should be portable ...

Update: code for OS X here. Pretty readable & hackable. You are not by any means required to include rocket launcher control in your code, this is just in case you're interested in trying it for 'fun.'

Exam III details

(Update: if you checked Moodle within the first ~10 minutes of my posting this, I may have been updating the overall grade calculations at the time. Anything after 02:35 is accurate.)

I finished grading your exams, albeit under somewhat savage time pressure these last days. That means I haven't had time to process the results very much, but your raw scores will be available on Moodle in the next half hour or so. Anyway, the results:

Average: 73%
Std. Dev: 14%
Hardest questions: 2,3,4

Based on these numbers, I will scale it a bit. Probably, this will mean adding something like 5-7% to your test grade, but I haven't decided that yet. I will scale it such that the class average is at the C+/B- border with a favorable distribution. Details of this should be clear by Friday ...

Despite the averge being a little lower than I had anticipated, I am happy with the results, having seen how you went about the problems. By and large, you know what you are doing, and it was mainly time pressure and simple things that tripped you up. We'll go over the solutions in detail today in class. As a peace offering for giving another bastard exam, we will also devote a good amount of time to rocket targeting and such things and less time to lecturing.

Random plots for your perusal:

Exam III grading

I hope to have the exams graded and back to you by Friday ... I would say Thursday, but I have a visiting grad student coming in tomorrow. 

In any event: you'll have them before the weekend if all goes according to plan. Don't freak out, it looks pretty good so far. I will scale the exams if necessary.

Most likely on Thursday we'll go over the solutions in class.

Exam III

Here it is. I'll try to have solutions out by the end of the week.

Exam III is IN 2 HOURS

Seriously, people have forgotten before.

Bring a calculator. And writing implements. You won't need anything else.

Have some coffee. Bring music (with headphones) if you think it will help.

Don't forget that you can ask me questions during the exam, and I am somewhat likely to give helpful advice.

Today's office hours

Today I have a meeting that cannot be avoided, and it overlaps the posted office hours ...

If you want to come by today, I'll be in my Gallalee office from 5pm until the review starts.

Solutions to Friday's quiz

Here is Friday's short quiz, and a solution key.

Exam review session

How about 7pm, Monday 13th April, in our normal classroom.

I'll go over some example problems and more details on what precisely will be covered. Come with questions.

HW 12 solution

Here you go. Let me know if you find any mistakes or have any questions.

Formula sheet, Exam III

Here you go.

Working on the HW12 solutions now, I'll post here when they're finished.

There will probably be a review session tomorrow (Mon) evening at 7pm ... I'll decide tonight and post details.

Studying for the exam ...

I hope to have the formula sheet for the exam and the solutions to HW12 posted by mid-afternoon tomorrow (if not sooner).

Last problem

the possible modes of the string have

f = (n/2L)sqrt(T/u)

where n is an integer, L is the length of the string, T its tension, and u the density. You know T=mg, tension is provided only by the hanging mass. You also know f is fixed by the resonator attached, f=120Hz. The only variables are m and n.

If the two given masses m1 and m2 work, but nothing in between does, then they must be adjacent harmonics - one is n and the other is (n+1).

Plug that into the above equation separately for both masses, set equal, and you can find n. Once you have n, you know everything in the equation above except u ...

Homework 12

Remember, today's homework can be handed in as late as tomorrow night without penalty.

Checking your code

In case you want to check that your code is working ...

If you use the drag equation

F_d = \frac{1}{2}\rho C A \vec{v}\cdot\vec{v}

with starting parameters

v_x=31\,\text{m/s} \\
v_y=35\,\text{m/s}\\
m=0.046\,\text{kg}\\
A=0.00143\,\text{m}^2\\
\rho=1.225\,\text{kg}/\text{m}^3\\
C=0.25

then you should get a range of 126 meters over level ground (and 221 meters with C=0, i.e., no drag).

16.25

You can find the velocity of a wave at any point along the string if you know the tension and linear density. At any point along the string, the tension is provided only by the rope below that point - you can't push on a rope.

Finding the time is handled just like the last problem in HW10 - t is the integral of dy/v(y), with the limits being the two ends of the rope.

16.34

The latter parts of this question will make more sense after tomorrow's lecture, if they don't from reading the chapter. To get you started:

The average power transmitted along the string is

P = \frac{1}{2}\mu v \omega^2 y_m^2

Here you don't know the velocity, but you can relate it to the tension and linear density.

If you superimpose two equal-amplitude sine waves together of the same frequency (so kx-wt is the same for both), but they differ only by a phase phi, the resulting wave is given by Eq. 16-51:

y(x,t) = \left[2y_m \cos{\left(\frac{\varphi}{2}\right)}\right]\sin{\left(kx-\omega t + \frac{\varphi}{2}\right)}

The amplitude then depends on the relative phase as well as the amplitude of each wave. If the waves are in-phase, the total amplitude is just double, but if they are 180 degrees out of phase, everything cancels.

Thursday's lecture

Ok, continuing with waves (Ch. 16/17) has won the poll ... so that's what I will talk about tomorrow.

That implies you should look over Ch. 16 and 17 before class if you can.

Standing waves in your shower

Important application of waves.

Research Work

If you like coding, and want to pick up some research work, let me know. Prof. Williams is in need of students to help out, particularly those who know some Java (though good coding skills are generally valuable, you can pick up the Java you need).

Cool factor: her experiment is a giant neutrino detector in Antarctica, see link above. (Pun intended.) Very exciting work, and you would have a chance to make a real contribution to a world-class international research project.

Let me know if you're interested, or contact Dr. Williams directly (via link above).

Current Grades

Moodle has been updated, your grades for HW10 and labs 7-8 are now there. Everything except HW11 is now up to date.

HW11 should be added by the end of the week, and HW12 over the weekend so that your grades are current going into next week's exam.

(FYI - Lab 6 = project memo; Lab 7 = sensor calibration; Lab 8 = group talks)

Thursday 9 April 2009

By the way: short quiz on Thursday. It will not present a problem if you have read Ch. 16.

Really, it will be easy. Just a heads-up.

Even-numbered problems for this week

Numerical answers for the even-numbered problems this week:

Due tonight:
15.24 x < 0.23m
15.56 T~2.0s; 18.4 N-m/rad

Due Thursday:
16.34 10W; 20W; 40W; 26W; 0
16.60 0.846 g/m

Random hints:

15.111 First, for the linear acceleration, you can always just apply the center of mass equations:

m a_com = F_net

Even though rotation is involved, the center of mass stuff still works.

For the second part, you want to find the net torque to get angular acceleration. You have a force F applied some distance x from the point at which the bat is held (point O), and that's it. The distance x is 2/3 of the length of the bat, given. This torque must equal the angular acceleration (alpha) times the moment of inertia about the end of the bat (so (1/3)ml^2).

Finally, if the bat is rotating after the hit, the angular acceleration implies that the tips of the bat have an additional acceleration in the tangential direction. If no one were holding the bat (or if it is held lightly, as the problem states), the rotation will just be about the center of mass. The tangential acceleration at the ends of the bat has to be (alpha)(L/2) in that case. In the end, this acceleration due to rotation acts in the opposite direction compared to the acceleration of the impact of the ball itself. At the "sweet spot" these two cancel exactly, and you shouldn't feel anything when you hit the ball - if you hit anywhere else, it 'stings' because of the net acceleration at your end of the bat.

In other words, there is (say) a forward acceleration due to the force of the ball hitting the bat, and a backward acceleration at the hands due to the resulting rotation. You'd like the two to exactly cancel for your force to be used most efficiently, which is just what happens at the sweet spot.

Thursday's class

So I'm torn about what to do on Thursday. Therefore, I'm going to let you vote - we can continue with what's in Ch. 16 & 17 (which is probably easier) or do one of two other topics which are a bit more challenging. All three are very relevant topics you *will* see again, so in that sense there is no wrong answer ...

We should continue on with wave motion, but we did cover enough that we could pick up the missing bits during dead week - what is left is mostly pretty straightforward. I suspect that if you read the rest of Ch. 16 and parts of Ch. 17 you can pick most of it up on your own - there is not a huge amount of depth there.

We could also go a little deeper into resonance, which we only touched on. This would involve looking at driven oscillations and expanding our investigation of damping. I think this option is more challenging, but probably a bit more fun.

A third option is to look at coupled oscillations. This is not unrelated to the outstanding material on standing waves, and would get into things like synchronization of oscillators, the speed of sound in materials, and Fourier analysis of complicated waves.

Only the first option is really covered in the textbook; none of these three would show up in any serious way on the final. Vote in the poll at right if you have a strong interest one way or another; I'll lecture on whichever topic has the most votes as of Wednesday evening.

Hint on the 2nd homework problem

When you have the torsion spring connected to the pendulum, you don't have any sort of simple equation you can use any more ... you have to just add the torques up.

First, you have a torque due to the weight of the hanging deal, something like (weight)(radial distance)(sin of inclination angle) if you're not into vectors. That pulls the thing back to equilibrium.

The torsion spring provides a little 'kick' in the opposite direction to keep it going. The torque provided is just proportional to the angle, (kappa)(theta)

In total you have then (weight)(radial distance)(sin theta) - (kappa)(theta) = net torque. The net torque must be the moment of inertia times alpha, the second derivative of angle with respect to time. If you use the low-angle approximation sin(theta)~theta, you can recover an equation for simple harmonic motion: angular acceleration is (omega)^2 times angle.

Twitter

The poll indicates that you are not wild about Twitter ... which I can buy.

For that reason, I won't post anything on twitter that isn't also posted here. That doesn't mean I won't post anything to twitter, just that you won't miss anything essential by not following it.

Today's bit of randomness

Physics is good for anything. Anything.

Exam III

As it turns out, you have an exam one week from today. The format will be much like the last exam - you will have a choice of problems (e.g., solve 5 out of 8), you can bring a formula sheet, you have 2 hours.

The exam difficulty will be somewhere between the homework problems and the example problems in the book (see below) - not as difficult as the homework (though similar to the non-bastard problems), but harder than the example problems. More details, including sample problems, will follow.

The topics covered are:

• 10.6-10 Rotation
• 11.2-4,6-11 Rolling motion, Angular momentum, torque
• 13.2-8 Gravitation (excluding central force laws, only the stuff in the text)
• 15.2-9 Oscillations and simple harmonic motion
• 16.2-8 Waves, pt. 1

Basically, chapters 10, 11, 13, 15, and 16, but not all sections are included. Owing to Honor's day this Friday (no class!), there will not be much time for review. Studying the homework solutions and example problems in the text will serve you well. I'll be online most of the weekend and available Monday before the exam (13 April) most of the day for questions.

There will be roughly 2 questions from each from:

• Ch. 10-11
• Ch. 13
• Ch. 15
• Ch. 16

If you have been understanding the homework problems, you will probably be fine.

Tomorrow's class

(1) Waves: read chapter 16 (or at least skim it)
(2) Problem-solving: homework for this week, more harmonic motion
(3) More rocket madness

Numerical answers

Preaching to the choir.

HW 11 Solutions

Here you go. They need some proofreading, but they are basically complete.

Note that for the first problem ScienceWorld already has a great solution up, so I just linked to it.

HW 12

Homework 12 is out.

Problems due Tues 7 Apr: 15.24, 15.56, 15.111
Problems due Thurs 9 Apr: 16.25, 16.34, 16.35, 16.60

We'll spend plenty of time during the week going over these problems before they are due.

Keep in mind that there is no class on Friday for Honor's day.

Homework via email

FYI, I've created a separate email account just for homework ... since my inbox tends to get flooded when homework is due.

I mostly plan on using this for next semester, since we are already nearly done and old habits die hard. It is active right now, and I will be checking it ... and it would make things more efficient for me if you sent your homework to this address.

However, you can keep using the normal email address for the semester if you would rather, it isn't a problem.

Today's recitation

Today, we'll have one more presentation, if all the group members are around ...

After that, we'll work on homework problems - both the ones from last night, and the ones due tonight.

HW10 solutions

Here you go, all but the last one. Hope to have that solution out over the weekend ...

HW 11: 15.106

Problem 15.106
a) Translational - 1/16
b) Rotational - 1/32
c) it is a proof ... show that acceleration is proportional to -(const) times position.

HW 10 solutions

Sigh, I only have the first two problems done so far ... I will be working on them tomorrow and hope to finish them up. Here you go.

At the very worst, I will scan my hand-written solutions tomorrow

Following twitter updates, if you're in to that sort of thing

I am putting out some homework hints via twitter now, just as an experiment. You do not need a twitter account to follow these updates.

(1) the twitter updates will also be mirrored as my facebook status. You can look at my wall on facebook and see the same stuff.

(2) go to http://twitter.com/pleclair periodically

(3) get an RSS feed of my twitter updates, then you can read them in your aggregator of choice. You can get a facebook application to bring RSS feeds to your facebook homepage, fyi.

(4) (update:) follow the little twitter-box on the right side of this page ... type "pleclair ph125" in the box and hit "set."

One obvious question is why bother with this. I don't know, actually - it may not be worth the trouble. My thoughts:

(a) it is easy to follow twitter updates on a mobile device (if you have enough text messages in your plan). this means you can get HW hints and stuff without having to be near a computer.

(b) it takes me like 5 seconds to make a twitter post, whereas even on the blog I tend to spend 5-10min or so composing a post. If it is easier for me, I am likely to do it more often (even under savage time pressure), and the more help you get. (The reason I switched to a course blog in the first place was speed & frequency over static web pages.)

(c) On the other hand, having this blog is probably plenty already. I may be overthinking this problem, or going toward too many channels of information.

Physics scholarships

If you are a physics major, it is not too late to apply for a scholarship.

Specifically, the E. Scott Barr scholarships have not yet been awarded. Tell your friends.

Twitter

So, I am on the twitter (@pleclair). I was thinking lately it might be useful for posting quick updates for class, e.g., HW hints or notifications of new assignments. (Though I joined for other reasons, mainly because all the cool kids were doing it).

Would this be a useful way to disseminate small bits of course-related info?

(N.B. - it is easy to use twitter to update your facebook status and vice versa. Joining is not the time sink I thought it would be.)