Back of the Envelope, Ph 101 Order of Magnitude Physics

2017 Spring Term, Caltech

Lectures: Mon, Wed 1:00-2:30pm in 201 Bridge.

Final exam cover sheet (with instructions) is here. Please download and study the cover sheet (and the Purcell sheet) before beginning the exam! Do not download the exam itself until you are ready to take it. The complete Final Exam is here.

Due 3pm Thursday June 8, 2017 for Seniors and Grad students.

Due 2pm Weds June 14, 2017 for Frosh-Juniors.

Final exam solutions are here.

Prof:   Sterl Phinney  [final]
        316 Cahill
        x 4308
        esp [AT]
	Office hours: Tuesdays 3-4pm  (316/319 Cahill)

TAs:	Tristan McKinney  [PS 3,5,7,8, final]
        tmckinne [AT]
	417 Downs

	Ke (Kevin) Ye  [PS 1,2,4,6]
        kye [AT]
        446 Lauritsen , x6623




The big picture: at the end of this course, you should be unafraid to estimate numbers you don't know, make approximations in equations, and figure out what physics is important in a given experiment or situation, and see whether an explanation or number is worth pursuing further. You will should also have understood a lot more physics. It is unlikely that you will learn much of this if you do not attend the lectures: estimation and approximation are at least as much art forms as a science.

Approximate class outline (may change depending on student and instructor enthusiasms...)

1       Estimation, Dimensional analysis, scaling
2       More scaling, Bulk Properties of materials
3       Properties of materials
4       Biomechanics and exercise, water waves
5       Sound waves and acoustics, the ear, musical instruments, recording
6       Weather, oceans and atmospheres, climate change.
7-9     student vote from topics below

-Economics, industry and finance.
-Bombs, guns, torpedos, nuclear reactors, supernovae and other things
 that go bang.
-Tricks for general ODE and PDE equations.
-Earthquakes and their effects.
-Birds, airplanes, helicopters, spacecraft, lift, drag, boundary layers,
 turbulence, Kolmogorov spectrum.
-Nuclear and atomic physics; cross-sections and reaction rates.
-Astrophysical objects: stars, planets, cosmology
-Biology: neurons, information processing. Evolution, metabolism, lifetime.
-Challenge me!

TEXTBOOK: None required, but highly recommended (depending on your interests and background) supplemental readings are:

Physical Fluid Dynamics
by D. Tritton
ISBN 978-0198544937 (2nd Ed)
(Oxford University Press)
Gases, Liquids and Solids
by D. Tabor
ISBN 978-0521406673
(Cambridge University Press)
Feynman Lectures on Physics, Vols 1-3
by Richard Feynman
ISBN 978-0465023820
(Basic Books)
Or read 'em free online
Street-Fighting Mathematics
by Sanjoy Mahajan (a former Ph 101 TA)
ISBN 978-0262514293 (3rd Ed)
(That Other IT Press)
The Art of Insight in Science and Engineering
by Sanjoy Mahajan (a former Ph 101 TA; book is based on an early version of Ph 101)
ISBN 978-0262526548
(That Other IT Press)



The "final" will contain approximately six problems posed by the instructors. You may optionally replace one of these problems with an original one you pose and solve by yourself. You may work on this problem before the exam. It may not be one you find solved anywhere, the solution must be entirely of your own making. If you wish to exercise this option, it is recommended that you ask Sterl midway through the term, and at least a week before the final, if your problem is an acceptable one. Your grade will be determined by both how interesting and original the problem is to Sterl and the TAs, how rich it is in physics, and how you elegantly you solved it by order of magnitude methods.

Final exams will be due 3pm Thursday June 8, 2017 for Seniors and Grad students. Due 2pm Weds June 14, 2017 for Frosh-Juniors.


There will be approximately weekly homework sets due at the beginning of Wednesday's class, one week after they are posted on this website (see homework link at left). It is your responsibility to access the problem sets and to turn them in on time. Even if you are on travel when a problem set is due, the miracles of modern technology (scanners, phone cameras, email) will enable you to get pdf's or jpg's of your homework solutions on time to the instructor and TAs. Just remember to reduce the resolution so the file size is reasonable (< 300 kbyte/page, < 5Mbyte total, to avoid rejection by campus email; note that campus email strips all .zip attachments; .pdf is ok).

To satisfy Federal FERPA privacy rules, please choose a 6 digit code or pseudonym, and e-mail this to the TAs before the first assignment is due. To simplify ordering manageable return piles, if you choose to use a pseudonym, preceed it by a digit: e.g. "1Spock", or "177625" [phone key equivalent], not "Spock". You will use this, and not your name, to label the assignments you hand in.

Solution sets will be posted on the website. Graded problem sets will be available in class or in the Ph101 box outside 316 Cahill. If your name, and not just your numerical code, appears on the assignment, you will have to pick it up directly from JoAnn Boyd in 321 Cahill between 7:00am and 4:00pm weekdays. You will have to show her a TSA and FERPA-approved Photo ID to collect your papers.


You may take one full-credit one-week extension on a problem set during the term. No need to contact the instructors or the TAs, just write it on your problem set. If that homework is more than one week late, or if more than one homework is late, it will not be accepted nor graded unless prior arrangements with Sterl have been made as described below. Other extensions will only be granted for extraordinary reasons ---documented health issues, family emergency, etc. You must consult one of the instructors in person or both by e-mail, before the homework is due. Some sort of proof of extenuating circumstances (e.g., certificate from the health center, counseling center, or Dean of Undergraduate Students) will be necessary. The one free extension in the previous bullet item is designed to satisfy other ``reasons'' such as ``I had too much other work'', so don't waste it and expect additional extensions for such reasons. In the event that Senior Ditch Day falls on a Monday, Tuesday or Wednesday, the homework that would have been due that Wednesday will instead become due on Friday of that week.


Your grade will be a mostly monotonic function of
g = [0.7(sum of homework scores)/(total possible) + 0.3(score on final)/(total possible)].
Because new topics and new problems are given every year, and the mix of students varies, Sterl determines the connection between letter grade and numerical score by looking at natural breaks in the final distribution: there are no fixed cutoffs and no predetermined curve. However as a very rough guide, in 2017 A+ was > 0.93, A was > 0.89, B was > 0.75, C was > 0.65. in 2015 A+ was > 0.82, A was > 0.75, B was > 0.6, C was > 0.45. In 2009 A+ was > 0.9, A was > 0.8, B was > 0.67, C was > 0.5. In 2007 A+ was > 0.9, A was > 0.85, B was > 0.75, C was > 0.6.


Collaboration policy on the "final":
  1. The "traditional" part of the exam will be closed book and closed computer-like devices, except for Purcell's sheet of useful numbers.
  2. if you choose the option to write one of your own problems to replace an exam problem, you may talk about general topics, reference papers and equations with anyone as you are choosing the problem and trying out ideas. But you should do all the final choice of problem, solving and writing yourself.

Collaboration on the homework is limited. You must first try every homework problem BY YOURSELF for at least 30 minutes without external help (human or internet), other than looking up fundamental equations (e.g. Navier-Stokes, Schroedinger, Planck black-body, etc) in the recommended texts. This is the fun, OoM part. For estimation problems, this in particular means you must work out an answer using only what is in your brain before typing anything into a search engine. If subsequent Googling suggests your estimate was way off, you can then try to understand what went wrong with your estimate (or, not infrequently, with the dubious web "information").

Visual exchanges of information are strictly forbidden -you may not trade equations, graphs, or look at other people's solution sets from this or any prior year, or from child or grand-child Ph 101-like courses at other universities.

You may consult books and published papers to learn or remind yourself of relevant physics. The recommended texts are (surprise) recommended.

If after spending 30 minutes on a problem you are still stuck on it, you may TALK about the homework with the TA or your fellow students, but all exchanges of information must be general in nature and either exchanged verbally, or with modern replacements for talking (i.e. texting, IMing and emailing is ok too, as long as details are avoided -see below). For example the following QandA is ok Q: "I got a density of one atom per cubic km. Isn't that awfully low for lead at room temperature and pressure? A: "Yup, sure is. What variables did you include in your Buckingham Pi list? Oh, I think quantum mechanics is relevant here. Why did you leave out hbar? The following one is NOT OK: Q: "I'm stuck on problem 2. Can you help me?" A: "Sure. You take equation 3.12 of this book, insert equations 2.5 and 3.2, integrate and you should get the right answer which is V k squared over pi squared".

After any discussion with others, you must write up your own homework by yourself, without reference to anyone else's.

In real research, no one else knows the answer to the problems you work on (otherwise why would you be doing them?), so the most important thing you can learn from homework is how to think and solve for yourself, and be confident in your answers.