40,808 reputation
356130
bio website inspirehep.net/…
location Duchy of Grand Fenwick
age 44
visits member for 4 years, 7 months
seen 32 mins ago

Experimental nuclear and particle physicist. These days I'm teaching, but I've spent a lot of time on nucleon structure in fixed target electron scattering and neutrino oscillations using reactors and beam sources.

Been reading Zemansky & Dittman's book on thermodynamics recently. Quote: ``The concept of temperature is rich in interpretations and levels of abstraction.'' Nice.


2h
comment Lagrangian perturbation theory
"Why can I not put this question up for bounty? There is no button for me to click!" It might help to check the help center topic on bounties. Short version: you need 75 rep to set a bounty.
2h
comment Arbitrary acceleration written as centripetal acceleration of approximating circle
What Aneek recalls it incomplete. It should be clear that an acceleration (anti-)parallel to velocity is not described by any instantaneous circle and nearly as clear that any acceleration transverse to the velocity can be. Not that this is much use in finding trajectories as the information needed is identical to that needed to simply describe the accelerations in the first place.
7h
comment Why does a change of direction imply an acceleration?
Notice that to an observer passing in a train moving at $10 \,\mathrm{m/s}$ you add $0.5\,\mathrm{J}$ to the mass, but still change it's velocity by $1\,\mathrm{m/s}$. (That is what I've written as (2) above.) The work done is not a frame-independent thing. The importance of invariance may take a while to get used to, but it deeply fundamental. There are frames in which any particular instantaneous change of direction of an orbiting satellite takes the form of a pure change in instantaneous speed: but that could be from stopped or to stopped so the KE change may have either sign.
13h
comment Why does a change of direction imply an acceleration?
Impulse is the right quantity to ask about and it comes directly from the impulse-momentum theorem. $\vec{J} = \Delta \vec{p} = m \,\Delta \vec{v}$, so that $|J| \approx m |v| \sin\theta$ (where the approximation is good for small angles). But frankly I think that your problem is that you are treating works like "force" and "energy" as if your colloquial understanding of them is good enough rather than treating them as having precise meanings. You shouldn't be guessing at which concept applies, you should be able to deduce which one to use.
13h
answered Why does a change of direction imply an acceleration?
1d
revised How is the formula for the capacitance of a parallel plate capacitor derived?
Remove apparent spam link
1d
comment On the application of chaos in signal processing
SKM: Daniels suggestion looks good to me, but we don't generally migrate to beta sites without the poster's permission. Odds are you'll get a better answer on a specialist site, while this question is hovering is the hazy boundary of off-topicalitiy on Physics. Just flag it and request that we migrate.
1d
comment Hafele-Keating revisited with a gravity clock
One can treat the precision of the perihelion of Mercury as a measure of the time it takes the planet to fall and rise in a gravitational field by comparison to the time it takes to go around (slightly complicated by the non-constant distance, alas). In that view the experiment has already been done and is consistent with General Relativity. That experiment can only achieve the necessary precision by running for decades at a time.
1d
comment Does an orbiting body accelerate?
@HolgerFiedler Seriously? Introducing a geometric notion of gravity to a OP still struggling with the basics of first semester mechanics is doing them a disfavor. In the treatment the OP is using gravity is a force and the orbiting body accelerates. Mark can grapple with the subtleties of relativity at a later date.
1d
comment Resonance - electron cloud - oscillation
"The exercises are usually based on the last lecture." That kind of meta reasoning is fine as far as it goes, but won't be much help once you get outside the classroom environment. So ask you 'What is the physical significance of the plasma frequency?" and 'Does that actually shed any light on this problem?' If the answer is negative then you need to explore the system on a different conceptual basis.
2d
comment How to actually determine the Index of Refraction of an object in different colors of light?
You can certainly look up the frequency dependence of various optical properties of materials in big dusty handbooks. Try the library of the nearest significant university or the internet. Incorporating that information into a graphics engine is not as easy as diddling a texture, however: you're going to have to trace rays, or work out canned solutions for simplified geometries and apply them using a heuristic. All of which is fascinating, but none of which is physics (well, aside from designing the ray tracer).
2d
comment How much wind power is needed to lift someone up?
The above considerations should also explain something about the design of the human-powered rotor-wings that @CuriousOne mentioned.
2d
comment How much wind power is needed to lift someone up?
One of the several problems with the framing of this question is that you are asking for power of an operation that intrinsically doesn't require any at all. What is required is force. Now, because you are hoping to generate that force as a reaction from pushing air around, there is an actual power cost, but how much depends on your thrust geometry: moving a lot of air slowly requires less power for the same thrust than moving a little air fast. But that brings us squarely into an engineering problem, not a physics one.
2d
comment Is it possible for two events happen at the exact same time?
My point was that "basketballs hitting the ground" is not in any detailed and reasonable understanding something that happens at a single point in time. It can only be treated that way in the highly simplified approach used in the first few weeks of introductory classes. By the time you have the concept of impulse it should be clear that the event can't be confined to a single mathematical point in time.
2d
comment Is it possible for two events happen at the exact same time?
Non-trivial, physically-realizable events generally require a finite period of time in which to happen, as well, so overlapping those periods doesn't even require arbitrary precision.
2d
comment Could a spaceship design incorporate several kinds of force or effect?
This is more or less explicitly an engineering question.
2d
comment Is there any basic physics textbooks that do not have reference to sport?
Note that there has apparently been an informal effort to remove the formerly numerous war-related references from textbook examples and exercises in recent year and many seem to have gotten replaced with sport related equivalents. You could try an older text. I also see books now that use the exercise formerly known as "shoot-the-monkey" without that name and often with out the simian.
2d
comment Can one verify the Earth tide in his yard?
As @Curious says, a fixed, integrating GPS station can achieve better than 20 cm resolution, so you ought to be able to pick the approximately diurnal variation out of the observations. The signal will be buried in a fairly noisy background, but it should be there.
Jun
27
awarded  photons
Jun
26
comment Relativity… Twin paradox
In particular if you follow the procedure in physics.stackexchange.com/questions/2554/… you can can answer your question for yourself.