31,437 reputation
340100
bio website inspirehep.net/…
location Duchy of Grand Fenwick
age 43
visits member for 3 years, 7 months
seen 2 hours 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.


Jul
4
comment Kinematics - moving body
So far, the question is underspecified. Is the plane accelerated by an outside force or is it moving freely in reaction to the weight of the block?
Jul
4
comment Bomb trajectory from an airplane
@TimSim It is the surface reaction (normal force) that causes the horizontal component of that acceleration.
Jul
4
revised How can time dilation be symmetrical?
blue pencil
Jul
4
comment Twin Paradox Without Acceleration
World lines. You must track the world lines because that is the only way to compute the proper time. I know every discussion of relativity starts by talking about relative speed but those methods are the hardest way to work these problems. The invariance of proper time is the easy way to work them.
Jul
4
comment Moving towards a clock at .866C
@Krel Your problem here is that you are failing to distinguish between the arrival frequency of the light fronts and the frequency with which the light fronts were generated. At significant relative speeds these can differ by a lot.
Jul
4
comment Moving towards a clock at .866C
Also related: Would approaching a distant star at near the speed of light unfold its entire history in “fast-forward”?
Jul
4
comment Moving towards a clock at .866C
Take note this rule applies to the timing computed after lightspeed delays have been figured in. For a fast approaching source the pulses may arrive more frequently then every second (and pulses from receding sources will come in even less frequently). Most intro texts have a section of "apparent velocity" where this is discussed.
Jul
4
answered How can time dilation be symmetrical?
Jul
4
comment Twin Paradox Without Acceleration
I suspect the reason for comments instead of answers is that physicists find this questions to be boring. The twin paradox has been fully explained for around 100 years at this point. It is still difficult to communicate why the explanation is true to the layman simply because Minkowski space is so very different from how people think the world works: "acceleration" is a cop-out to avoid the hard work, but the hard work can't be done in a page or two if the audience doesn't have the preparation (because first you must explain how the world actually works and they won't believe it).
Jul
4
comment Neutron Diffraction Device
In practice neutron diffraction is general done with as mono-energetic a beam as you can arrange.
Jul
4
comment Swinging onto a Hill
For ski jumpers the design of the jump mostly controls the landing geometry. In the scenario the OP offers the victims have pretty minimal control. I suspect that for many cases the naive answer is correct here simply because without skis you can't take maximum advantage of the ski-jumping behavior. "You want me to bend my knees and roll after falling how far?" kind of thing.
Jul
4
comment Swinging onto a Hill
This is the naively correct answer, but note that ski jumpers land safely on a hill that slopes away from them after making every effort to fall as far as possible, and part of what makes that possible is hitting the snow with the largest component of their velocity directed along rather than into the surface.
Jul
4
comment What is the maximum value of the frequency of oscillacion for which the coin will stay on top of the block?
When faced with a problem like this, envision the situation described. How might the coin behave? Ans: it either sticks to the block or slides around; if it slides it will jiggle about until it eventually falls off. So now you have reduced the question to one of whether it will stick or slide...
Jul
3
comment Spherical shell with Mass of the Earth
In all honesty I was (and am) trying to help. Not necessarily help with this question but help you to be a better physicist in the future.
Jul
3
comment Spherical shell with Mass of the Earth
The math you have written down is very nearly complete and the last step is trivial if you understand the physics you are doing. Unfortunately the fact that you don't see that doesn't tell us what you don't understand; just that you have missed something and are treating the math as part of a cookbook recipe. At what point do you not understand the physics behind the equations that you have written down? That tells you what your conceptual question is.
Jul
3
comment Spherical shell with Mass of the Earth
I've been hashing this out with a couple of students lately so let me try to elaborate. You don't solve physics problems by know what mathematical steps to take. You solve physics problems by understanding the meaning of the physics and building a chain of reasoning from where you are to where you want to go. Writing down the math should be the easy part (though sometimes you end up writing down a hard math problem), and once your chain of reasoning is complete finishing off the math should be a mostly mechanical business.
Jul
3
comment Spherical shell with Mass of the Earth
To the reason for the closure (or at least the reason I'm not surprised by the closure; I don't speak for anyone else): your question "BUT WHAT THEN" could be reasonable translated as "do the next step for me". The physics associated with the equations you've written down are enough to finish the problem, so we have no idea why you don't do so. My suspicion is that you haven't fully understood the meaning of something here (be it in the question or in the steps that you have taken down from the text or your notes), but we are left guess what that might be.
Jul
3
revised Spherical shell with Mass of the Earth
latexify; restore homework tag
Jul
3
comment Spherical shell with Mass of the Earth
The "homework" policy applies to questions that are homework-alike without requiring that they were assigned to you as part of a class. Also, we use MathJax to supply LaTeX-alike typesetting of mathematical expressions; I'll do this one for you by way of an example.
Jul
3
comment Ideal gas and inelastic collisions
Without internal degrees of freedom you have a simpler model and can develop the kinetic theory of gasses without distractions. Indeed this is usually done in a intro-level course.