24,249 reputation
155129
bio website lightandmatter.com
location Fullerton, California
age
visits member for 3 years, 3 months
seen 15 hours ago

I teach physics at Fullerton College, a community college in Southern California. I have an undergrad degree in math and physics from Berkeley and a PhD in physics from Yale. Back when I was doing research, my field was experimental low-energy nuclear physics.


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comment Black hole “no hair” theorem
Electric charge isn't quite as special as you're thinking. The no-hair theorems you usually hear about are for electrovac assumptions. So the special role of electric fields as opposed to any other fields is put in as an assumption. There are known counterexamples if you allow other fields. See livingreviews.org/lrr-1998-6 .
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comment Why GPS is at LEO?
@JerrySchirmer: That's not quite right. The usual goal is to locate yourself on a map, i.e., horizontally, and for that you get the best accuracy when the satellites are close to the horizon.
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comment Sound diffraction through a single slit
I wasn't the person who flagged it as low quality, but the original answer was only one sentence.
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comment Sound diffraction through a single slit
I don't think this is right, for the reasons given in my answer. Note that I gave three separate mechanisms for the diffraction to behave differently.
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comment Sound diffraction through a single slit
@Floris: You could also worry about the dielectric constant of the material, etc. You can't just assume a material with no electromagnetic properties -- such a material would be perfectly transparent.
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awarded  Cleanup
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revised Sound diffraction through a single slit
rolled back to a previous revision
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revised Sound diffraction through a single slit
added 247 characters in body
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answered Sound diffraction through a single slit
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comment Sound diffraction through a single slit
related: physics.stackexchange.com/questions/5886/… physics.stackexchange.com/questions/141562/…
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comment Recommended books for undergraduate electrodynamics
@ChrisWhite: No way. Jackson has much more depth and breadth than Purcell. I learned E&M from Purcell as an undergrad and had Jackson in grad school, and there was a ton I learned from Jackson.
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answered For a massless pulley moving upwards with acceleration, is the upward force equal to the downward force?
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comment For a massless pulley moving upwards with acceleration, is the upward force equal to the downward force?
The force equations you provided are wrong. He's applying Newton's second law to the pulley, which is perfectly legitimate.
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comment For a massless pulley moving upwards with acceleration, is the upward force equal to the downward force?
I see. I edited the question a little and retracted my close vote.
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revised For a massless pulley moving upwards with acceleration, is the upward force equal to the downward force?
clarify based on comments
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comment For a massless pulley moving upwards with acceleration, is the upward force equal to the downward force?
Clearly, since there is a net upward force, the pulley itself will accelerate upwards. Huh? Why doesn't the whole apparatus just drop?
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comment If something is not moving in space, is it moving on the time axis at the speed of light?
This is why people say "if something is not moving in space, then it is moving on the time axis at the speed of light." "People" is the popularizer Brian Greene. Relativists in general don't talk this way.
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comment For a massless pulley moving upwards with acceleration, is the upward force equal to the downward force?
What do you mean by "not attached to a ceiling?" Are you saying that the whole setup is just free-falling straight down? If it's not attached to a ceiling, then what object is exerting the force F on the pulley?
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comment What is logical way to calculate percentage error?
I disagree on both counts. There is no particular reason to prefer division by A or by B in general, and it is not true that we don't care about the sign. As an example where you would certainly not want to divide by $B$, there could be a case where you're testing a theoretical prediction that $B=0$.
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comment What is logical way to calculate percentage error?
Percent error is almost never of interest, so the real answer is that working scientists would never worry about this issue. If you're testing an experiment against theory, there's no way to know whether a 0.03% difference is consistent with the theory or inconsistent with it, because it depends on how much error would have been expected due to the inherent precision of the technique. In real science we would say we measured A=____$\pm$____, and compared with the predicted value B=____ this was off by, e.g., 5.7 std dev, which is highly statistically significant, so the theory is disproved.