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Jan
26
comment Order of the life time of the K± mesons
Deleting and re-posting a question because it is drawing downvotes or close votes is not appropriate. Fix the existing question.
Jan
26
comment Power vs. Speed - Indoor rowing
Two things. First, power goes by $v \cdot F_{drag}$. Second, recall that the thing you're trying to model is a boat, so in addition to Plank (simple momentum transfer, $F_{drag} \propto v$) and Prantl (viscous friction; $\propto v^2$) drags there is a wave-making term ($\propto v^3$) and eventually turbulent drag (still higher powers). The problem is complicated.
Jan
26
comment Clarification on conservation of energy for (or internal potential energy of) $N$ particle system
"Is it just the weak form of 3rd law of motion?" Yes.
Jan
25
comment Is muon muon annihilation already realised?
I imagine the odds are pretty decent that the process is actually been recorded in some LHC event from one for the detector packages. But no one is looking for it there and so we probably don't know it.
Jan
25
comment Does a rapid temperature change crack annealed glass
Related: physics.stackexchange.com/questions/57482/…
Jan
24
comment Will CMB images change as technology progresses?
Back in the early 90s we thought the COBE pictures were the bee's knees, and WMAP put those to shame. The Planck data is just fantastic. But the interesting question just at this moment is one of funding: JWST is sucking down huge amounts of money and making things tight for everybody else.
Jan
24
comment What prevents an atom's electrons from “collapsing” onto its protons?
@GunDeniz John has left it out, but you can perform a standard transformation to the center of mass coordinates and get an effective problem with a particle of "reduced mass" $\mu = (m_em_p)/(m_e + m_p)$ moving in a fixed field. In the case of hydrogen $\mu$ is very nearly equal to $m_e$, though careful spectroscopy of protium (normal hydrogen) and deuterium (heavy hydrogen) can see the difference in the reduced masses.
Jan
24
comment What force keeps electrons in their orbitals and not collapse into the positively charged nucleus?
physics.stackexchange.com/questions/20003/…? physics.stackexchange.com/questions/9415/…? and many others. But it might also be worth noticing that the 1s orbital has it's maximum probability in the nucleus.
Jan
23
comment When is a negative focal length used in optics?
"If an image appears on the same side of an object, then we use a negative focal length for determining information about the image's distance," is correct for lens and incorrect for mirrors. The sign conventions of ray optics are detailed and have to be followed with care for everything to work out.
Jan
23
comment Dot product and divergence
Thinking about the meaning of these notations (and that for the gradient) will start you on the road to understanding the algebra of differential operators.
Jan
23
comment Do metal shavings affect the magnetic field they are used to visualize?
Not sure that physics.stackexchange.com/questions/41025/… is the one I was looking for, but it is relevant.
Jan
23
comment Do metal shavings affect the magnetic field they are used to visualize?
I would swear that we have a closely related and highly voted question on the site already, but I can't seem to run it down.
Jan
23
comment Kirchoff's law, can it tell me the direction of current in this case?
While you can suss out the answer for these trivial circuits with this kind of ad hoc thinking there is no general way to look at the topology of a network and know the direction of current flow independent of the values of the components. The saving grace is that it doesn't matter if you draw the current the wrong way because working out the math will then give you a negative value for that segment.
Jan
23
revised NO Uncertainties for particles in their own frames!
deleted 1 character in body
Jan
23
comment Does relativity violate uncertainty principle?
Related (duplicate?): physics.stackexchange.com/questions/111970/…
Jan
23
comment Would a grid of $10^{57}$ hydrogen atoms collapse?
That's a bit under a solar mass, and you've proposed a density (1 billion atoms per cubic meter) much higher than that observed in a typical nebula, so you are likely to get some kind of compact body, but the dynamics of nebular collapse are complicated, and I won't venter a guess as to how much mass will be lost.
Jan
23
comment Orbit of a planet
Even in classical mechanics there are only exact circles in the strict two-body case. The universe has more objects than that. Draw your own conclusions.
Jan
23
comment Measuring force of a punch
Why much around with accelerometers when the usual tools for this kind of thing are pressure transducers? Or are you trying to rig someway to use your phone as the data acquisition platform?
Jan
23
comment Measuring force of a punch
Couple of comments here. First and foremost, the delivered impulse is certainly depends on the resistance of the target because the punch is not delivered as the free flight of an object and the kinetic chain is involved in any punch of consequence (indeed I would argue that any attempt to analyze punching in terms of physics 101 concepts is doomed to dismal failure). Secondly, the torque necessarily imparted to the object is almost certain to ground the opposing edge and render the measurement imprecise even for what it is. It's a reasonable starting notion, but the devil is in the details.
Jan
23
comment Long distance radiation detection, David Hahn and the clock
Note that radium sits at the top of a length decay chain, so it (a) has a high specific activity and (b) the chain emits more than a little gamma which gives you as much range as a passive source like that will allow. Now, you can still ask "How slow was he moving and how close did he have to get?", but the problem is easier than with many other radioisotopes.