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- March 28, 2004 at 3:40
2 Posts

What are the *most unclear* explanations of the
physics curriculum you experienced?
Which basic questions are left most
unsanswered in you eyes?

I would add such topics in the next (16th)
version of the free 1100 page physics
textbook freely downloadable at

   Christoph Schiller

Replies to

Hamiltonian - March 30 2004 3:46
2 Posts

We wants to know why people prefer to use a nassty Hamiltonian when a perfectly good Lagrangian will do.

Hamiltonian - April 01 2004 8:06
1 Posts

Well, this is probably the standard argument but here goes.  I think the hamiltonian formulation has a subtle difference from the lagrangian formulation in that the free variables (qi's and pi's) are now indepenently allowed to be varied.  in the lagrangian of course you have q and qdot which are not indepent.  Also in introductory quantum mechanics it is easier to formulate the shrodinger equation in terms of a hamiltonian operator at first.  There will come a time where we actually move back to the lagrangian, i.e. when you begin to study field theories and such.  Feynman's lagrangian formulation is very popular in that subject.  I dont know if this helped but i hope it did a little bit.


Hamiltonian vs. Lagrangian - April 09 2004 10:56
Dave Avatar
San Marcos, Texas
431 Posts

A good discussion of the relative merits of the two formulations is discussed in Thornton and Marion, chapter 7.  If you are interested in a more detailed discussion, check out chapters 8, 9 and 10 of Goldstein (the standard graduate text for classical mechanics).

Try not to become a man of success, but rather try to become a man of value -- Albert Einstein

It's good for something - April 14 2004 9:53
1 Posts

Hamiltonian is good for determining constants of motion in systems.  Using Poisson brackets and some other nifty stuff, constants of motion can easily be found.
This is also useful in chaotic systems.  Study of Hamiltonian chaos uses the Hamiltonian stuff and action angle variables as "variables".
So, it is good for some stuff.  Just maybe not the good ol' classical stuff.  (L'eqns are better to me too)

- March 30 2004 11:31
4 Posts

I came across something while tutoring some physics students, that torque and energy have the same units (N.m or Joules).  I never noticed this before, and I am not sure what it means.  Is torque a form of energy?  Why do we use the units of N.m for torque instead of calling them joules?

Torque and Energy - June 22 2006 3:17
Spyridon Koutandos
1 Posts

My name is Spiros from Greece.I believe that the explanation of quantum mechanics gives an answer to your question.Let me first explain that the explanation of quantum mechanics is kept secret within the MENSA club. I do believe that in nature there exist mostly Pressures and torques , while the concept of force is kept only for gravity.In quantum mechanics this is declared by h bar, bar is units of pressure, and psi the wavefunction is pounds per square inch. I still havent found how torque is included in quantum mechanics.Contact me if you have an idea.

torque and energy - March 31 2004 8:44
Society of Physics...
293 Posts

Yeah, I've always thought this was cool...I think of it as nature's way of of being efficient (using the same units to do two different things) or perhaps reminding us of the limitations of traditional dimensional analysis...In some real way I think it's probably best to think of the units and the mathematical nature of the quantity (vector, scalar, tensor, pseudo-vector, etc.) together when using dimensional analysis, so Joules are scalar N*m's and torque is measured in vector Nxm's...

Adjunct Professor of Physics, Editor of The Physics Teacher, and GWU SPS Chapter Advisor

tides - March 31 2004 8:49
Society of Physics...
293 Posts

most texts do a terrible job about explaining the tides...another huge weakness is in the explanation of how batteries work...

Adjunct Professor of Physics, Editor of The Physics Teacher, and GWU SPS Chapter Advisor

The adventure of physics - on 1300 free pages - May 25 2006 8:41
2 Posts

I worked the previous comments into the new,
nineteenth edition of my free Motion Mountain physics text.
It can now be downloaded at


Over 1300 pages lead through the whole of physics, from mechanics to
relativity, electrodynamics, thermodynamics, quantum theory, nuclear physics
and unification.

The nineteenth edition gives details on radiometric dating techniques,
explains why it is forbidden to carry thermometers on airplanes,
shows how to use the same
idea to measure the speed of bullets and that of light, presents a classical
system that obeys the Schroedinger equation, introduces Tesla coils, shows
that space-time has different properties in certain patent offices, tells how
to see effects from atoms using only a lamp and a piece of metal, gives more
details on clouds and jets of astronomic size, shows how to perform a
precision Michelson-Morley experiment, gives the latest results on the Pioneer
anomaly, introduces fusion reactors, demonstrates the chromatic lens errors of
the eye, and presents the simplest unsolved problem about the trefoil knot.

The text also provides improved writing, more figures, more curiosities, many
additional solutions to the challenges, and, thanks to the help of Martin
Elsaesser, the first embedded animation. Many thanks also to all those
readers who have suggested improvements and material for the text. An errata
page that allows direct feedback via the website is now available.


Christoph Schiller

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