JustJeff
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# 28 Comments

 Nov 3 comment Is energy expended when a force is exerted on a object? no work is done on the object -- which is not the same as saying that which exerts the force does not dissipate energy. Sep 27 comment Why do car keys have longer range when held next to your head? RF key fobs are typically only somewhere around 300 to 500MHz, for which the wavelength is 1m down to 60cm. Even at 2.45GHz, common for lots of things but not afaik, key fobs, wavelength is ~ 12cm. Sep 29 comment What is the relationship between AC frequency, volts, amps and watts? hit 'flag' like I just did. A mod will come and see these comments and that ought to do it, I'm pretty sure they have an easy button. Sep 26 comment What is the relationship between AC frequency, volts, amps and watts? might be better served on electronics.stackexchange.com Sep 19 comment How to find the poles on a spherical magnet? the torque being proportional to the sine of the angle is problematic, though, in that it wouldn't take much to influence where the equilibrium point lands. the plate should be smooth, as should the magnet, to reduce any kind of friction that would prevent rolling. Any kind of field in the plate would hugely skew the result. Sep 19 comment How to find the poles on a spherical magnet? assume the sphere is in contact at some point other than where the flux density is max (i.e., the pole). The distribution of the field around that contact point will be asymmetric. In particular, the region of maximal flux density should attract the plate more than other points at the same angle away from the contact point. Most of this force would be upward, but there would be a slight tangential component (going as sine of the angle of contact-center-pole) that should torque the sphere so as to roll until the pole produces no such torque, as happens when the poll IS the point of contact. Jun 19 comment Is fire matter or energy? fission, fusion, and ordinary combustion ('fire') are all processes. Jun 19 comment Is fire matter or energy? +1 just on the basis of naming it for what it is, a process. May 8 comment stability of hypothetical lunar atmosphere @anna v - i've heard it said that the atmosphere of Mars is so thin, b/c that body doesn't have enough magnetosphere to keep the solar wind from interacting with the topmost reaches of its atmosphere. Presumably this effect would be more pronounced on the moon, w/weaker gravity still. I have no idea how one calculates the effect. May 8 comment stability of hypothetical lunar atmosphere just wondering whether it'd be days or millenia. Apr 28 comment What is the mathematical nature of space time quantization in string theory/super string theory? string theory doesn't achieve quantization of space. string theory is 'background dependent', a description of how things happen IN space, taking the pre-existence of some kind of space as a given. much effort in string theory is spent arguing the shape and dimensions of that space. perhaps you are thinking of loop quantum gravity, a background independent view (like GR), which seems to take quantization of space itself as a starting point. Mar 6 comment Cosmic ray hazards so .. basically, the secondary particles produced from the initial collision are themselves high energy particles, and the vast majority of the particle shower simply keeps on going, through the astronaut, and out the other side? Feb 28 comment what does 'same energy as a ton of TNT' mean? for those tempted to reflexively LMGTFY -- isn't part of the mission of these stack-exchanges to become a repository for specialized content, vetted by a more knowledgeable user base, than one might get with a generalized wiki? the idea being that search engines will eventually point here? Feb 22 comment Dynamics of moment of inertia Not sure what L and V were about, but the meaning of last two equations is clear. Thanks! Feb 21 comment Dynamics of moment of inertia the problem here is what happens when gamma is a function of time, or of the angular position of the system. i believe what you have is correct for constant gamma, though, as it looks exactly like the math for dealing with transformed impedances in the electrical domain. Feb 20 comment Why do car keys have longer range when held next to your head? try holding it a half-wavelength from your head (reinforcing reflection) and compare that to holding it a quarter-wavelength (interfering reflection). Any difference? Feb 16 comment Visibility of moon-based laser from Earth @fortran - yeah, no doubt. So assume nice clear skies w/little pollution. Feb 16 comment Visibility of moon-based laser from Earth i think you've convinced me. conclusion - you'd need ridiculously stupendous amounts of power, and even then you'd just be changing the tint of one spot, which could easily be missed. so, if you wanted to undeniably prove to people 'hey, someone is mucking about on the moon', you'd probably have better luck as BarsMonster suggests, using a large Xe-flash. Feb 16 comment Visibility of moon-based laser from Earth yeah, it's how to think about how the laser (or Xe lamp) competes with the diffuse scattering that stumps me. fwiw, i only said laser b/c i figured the inherent directionality would deliver light on target better, but if you start from input power, you're probably right about the efficiency. Jan 17 comment Reaching speed of light obviously something's going to stop you from getting to c, but what about getting to interesting fractions of c, say, just enough for relativistic effects to become noticable?