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Jan
31
comment Boeing 737 Homework Problem
It's good that you showed work, but that's not all: "It's not enough to just show your work and ask where you went wrong. If you just need someone to check your work, you can always seek out a friend, classmate, or teacher. As a rule of thumb, a good conceptual question should be useful even to someone who isn't looking at the problem you happen to be working on."
Jan
31
comment Cesium-137 From Fukushima Meltdown
This is really something a biologist should answer, the key term being "biological half life." Moreover, there's a concentration effect -- e.g. iodine is concentrated in the thyroid, but something like cesium isn't. In the end though, the answer is there is absolutely negligible radiation from Fukushima and it will never have a measurable effect on your health.
Jan
30
comment Shapes of planets?
Possible duplicate of Why are stars, planets and larger moons (approximately) spherical in shape (like, the Sun, the Moon, the Earth, and other planets)?
Jan
30
comment What is torsional vibration?
The video linked in this tangential answer shows bending and torsional modes for a tuning fork.
Jan
29
comment How Special Relativity causes magnetism
Actually, the asymmetry has everything to do with physics, given that there are only electric and not magnetic charges in nature. Moreover, lots of the apparent symmetry between electricity and magnetism is an artifact of a 3+1 spacetime -- magnetism would not be expressible as a vector field in any other number of dimensions.
Jan
28
comment Group and phase velocity - why can the latter be faster than light?
@marmistrz With Fourier analysis, the sines and cosines are the entire functions, not just pieces of them. If you want to represent something that vanishes outside a certain region (even if it looks like a sine wave where it doesn't vanish), you need all sorts of cancellation to happen in the sum, and in fact you can only get the cancellation you want in the limit of infinitely many terms.
Jan
23
comment How can we see planets thousands of light years away but don't know if there are more planets in the solar system?
@Owen No, because you don't need or want to zoom into the object. No telescope on Earth can resolve a planet around another star, so we don't sacrifice field of view to try. On the other hand, it would take many nights to image the sky at even 20th magnitude with a dedicated telescope, and 28th magnitude is 1600 times fainter.
Jan
23
comment Why Vehicle's Engine back wheel driven?
If you think most cars are driven from just the back wheels, you clearly don't live in an area that gets snow ;)
Jan
19
comment Have cosmic rays and the CMB affected Earth's history?
Cancer: is that all that comes to mind when you think of mutations? Remember, evolution itself is seeded by genetic diversity. (A good biology question is how much comes from cosmic rays vs. copying errors.) Also, "causality" has a very particular meaning in physics. It is about the absolute limits for how abstract events can affect one another (often in the context of relativity). Something merely causing something else is... all of physics, really.
Jan
19
comment Does all hydrogen originate from the Big Bang?
@EdYablecki Except the hydrogen in your body hardly remained neutral (or monatomic) between the early universe and today, so if you think post-recombination hydrogen is different from that of pre-recombination, you must also think the hydrogen in you has nothing to do with that initial batch of neutral atoms.
Jan
18
comment rocket vertical landing
Or better yet Space Exploration.
Jan
17
comment What does it mean for a map to be “coadded”?
It's definitely a common term. It's used in pretty much every survey, whether the target is the CMB, galaxies, or supernovae.
Jan
16
comment How did people think of the universe before Hubble?
Might this be better suited for History of Science? In any event, the time before Hubble accounts for most of human history, and in fact most of scientific history, so this will need a long answer indeed.
Jan
14
comment Redshift due to a static gravitational field and the conservation of energy
Discussion on meta: meta.physics.stackexchange.com/questions/7425/… cc/ @AccidentalFourierTransform
Jan
13
comment What is the metric tensor for?
@Prahar The components (you know what I mean). Certainly a metric is never associated with a coordinate transformation.
Jan
13
comment What is the metric tensor for?
A metric is usually associated with a particular coordinate system. Can you elaborate on how you are using one in a transformation? Usually that would be done by the Jacobian.
Jan
11
comment Could the Big Bang be the result of a decrease in entropy?
Indeed this is not all a duplicate.
Jan
11
comment How is the perceived quality of sound measured and analyzed?
That comment made things more confusing. Before, I would have said the words you are looking for are "timbre" and "spectrum." But how does an instrument getting sharp as the temperature changes make it "worse"? Is it worse because it is no longer in tune with whatever reference you have?
Jan
11
comment Why pseudo-Riemannian metric cannot define a topology?
The claim is probably more accurately that pseudo-metrics cannot induce anything useful by considering balls (the standard way of inducing the usual topology on $\mathbb{R}^n$ from the usual metric). See for example here.
Jan
10
comment How to determine the radius of curvature of a convex lens?
@AccidentalFourierTransform Yes, I too have seen others use that reason. This may be revisited when we (eventually) revamp the homework close reason, but for now my opinion is that closure should be for objective violations of delineated policies. See also this.