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 Jan 14 awarded Taxonomist Nov 22 awarded Yearling Apr 28 comment Does average lifetime even mean anything? Since the probability of decay in a differential integral is constant, any time you find an radioactive particle, it will have the same average lifetime. In other words, if you take a collection of particles with average lifetime 5 minutes, then 5 minutes later all the remaining particles will have an average lifetime of 5 minutes from that point onward. Apr 13 comment Good way to compute the force of a hammer blow? Sounds tricky to me! My initial thoughts are that measuring momentum transfer would be much easier (e.g. hit a metal ball hanging from a ceiling). However, as you say, the force changes throughout the impact, is hard to measure and is very situation-specific. Depends what you need it for, but it would probably make most sense to estimate the collision time by other means, and talk about the average force as momentum transferred divided by time Nov 8 awarded Notable Question Oct 4 comment Martian space elevator from the vulcano Olympus Mon to the moon Phobos? Probably worth mentioning that Kim Stanley Robinson in his Mars trilogy [SPOILERS] decided to position the space elevator between Pavonis Mons (which is very close to the equator) and a smaller asteroid captured for the sole purpose of this enterprise. Phobos and Deimos are larger, and were more suited for other needs (for example military) Sep 20 awarded Popular Question Jul 2 awarded Curious May 31 comment Why are hydrogen energy levels degenerate in $\ell$ and $m$? I've seen that before, but I was wondering if perhaps there is a simple, descriptive explanation? Jan 2 awarded Notable Question Oct 4 comment Why an extra path difference of wavelength/2 is added when light reflects? physics.stackexchange.com/q/32122 Oct 4 asked Static pressure of free air stream Mar 17 awarded Popular Question Dec 18 awarded Popular Question Aug 30 comment Is it safe to ignore derivatives of velocity w.r.t. position and vice versa? The distinction between partial and total derivatives is rather crucial here. If it was total derivatives we were talking about, your approach would be right. However, by definition, partial derivaties ignore any dependance other than explicit! Aug 29 asked Digital camera as a measurement device Aug 28 awarded Commentator Aug 28 comment Is it safe to ignore derivatives of velocity w.r.t. position and vice versa? Are you sure the book didn't use partial derivatives? It looks like classic lagrangian thing to me Jun 7 awarded Nice Question Apr 3 answered Books that develop interest & critical thinking among high school students