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I write code in C++, Python, Java and now Haskell.


Jul
2
awarded  Curious
Jan
27
awarded  Notable Question
Dec
24
comment Precision of spectroscopy for astronomy
Also, I guess that distance from the source to the instrument has little impact? Or does it?
Dec
24
accepted Precision of spectroscopy for astronomy
Dec
24
comment Precision of spectroscopy for astronomy
It's fantastic!
Dec
24
comment Precision of spectroscopy for astronomy
Extremely impressive. But... if I a have start such as Kepler 11 moving around the COM of the system at about 1.3 m/s, isn't there the possibility of a star quake, or dark matter between Kepler 11 and the apparatus, or atoms moving in the apparatus that would make such a measurement very, very difficult?
Dec
24
revised Precision of spectroscopy for astronomy
added 60 characters in body
Dec
24
asked Precision of spectroscopy for astronomy
Nov
10
revised Motivation for form of Lagrangian
added 614 characters in body
Nov
10
asked Motivation for form of Lagrangian
Nov
2
awarded  Nice Question
Oct
13
accepted Friction forces and sliding slabs
Oct
2
comment Friction forces and sliding slabs
There are a few simple cases that should be easy to verify: if all the friction coefficients are zero, it should be possible to pull a slab without having anything else move, for example. Also, if only one slab is pulled, with less than required by the smallest friction coefficient, nothing should move.
Oct
1
comment Friction forces and sliding slabs
I'll put it in Mathematica and try it for a couple of cases where I think I know the solution. Thanks a lot!
Oct
1
comment Friction forces and sliding slabs
Hmmm... still something rubs me the wrong way :-) Suppose there is no friction, block i would move without i-1 or i+1 moving. Therefore, $F_{i+1}$, opposing the relative motion of i relative to i+1 should be directed towards the left, shouldn't it? Does $F_{i+1}$ comes out towards the left (negative) when using your equations, with only $P_{i}$ non zero, and all $\mu_{i}$ very small or even zero?
Oct
1
comment Friction forces and sliding slabs
In my case, $F_{i}$ is known, and I want the accelerations of each slab.
Oct
1
comment Friction forces and sliding slabs
Another issue is that any $F_{i}$ might not be strong enough to "break" a given interface, so IMHO it's possible that some accelerations are zero - in which cases all the slabs below do not move either, right? Shouldn't the final acceleration equations include some kind of branching depending on various cases (different relative values of $\mu_{i}$, ...).
Oct
1
comment Friction forces and sliding slabs
@ja72 - yes, I have no problem with the derivation, and I see how the content of W is used, it's just that the letter W doesn't appear by itself :-) No big deal. I would have stated N first, then F, then the equations for the acceleration. Also, the $\mu$ can change from static to dynamic, so that there are separate cases to consider, right?
Oct
1
comment Friction forces and sliding slabs
Staying stationary feels incorrect to me: first, in the case of 2 slabs, if the bottom slabs was moving in unison with the top block when F was not strong enough, why would the bottom slab suddenly stop? Second, even if an interface "breaks", it doesn't mean the friction coefficient there drops to zero. It changes to the kinetic friction coefficient, and there is still a force transmitted to the bottom slab via that friction. So, there are forces on the bottom slab, and hence possibly an acceleration. IMHO the bottom slab becomes stationary only if the forces on it precisely cancel.
Oct
1
comment Friction forces and sliding slabs
Can you explain a bit more how that works in terms of directions of the friction forces? I see that you are using Pi - Fi + Fi+1 = mi d2(xi)/dt2, which seems to assume that Fi+1 has the same direction as Pi. Is that always going to work?