Timeline for Having Trouble With The Principle Of Conservation Of Momentum For a Multiparticle System
Current License: CC BY-SA 3.0
12 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Sep 4, 2012 at 8:28 | vote | accept | alok | ||
| Aug 26, 2012 at 11:30 | answer | added | Emilio Pisanty | timeline score: 1 | |
| Aug 26, 2012 at 11:17 | answer | added | alok | timeline score: 1 | |
| Aug 25, 2012 at 18:42 | comment | added | Qmechanic♦ | @alok. If you now see it, you are welcome to write an answer to avoid creating an orphan post. | |
| Aug 24, 2012 at 13:48 | comment | added | alok | oh yes.Now i do. | |
| Aug 24, 2012 at 12:13 | comment | added | mmc | @alok I'm not sure what specific step is troubling you, but I'll take a guess. Do you see that $\sum_{\alpha}(\sum_{\beta\neq\alpha}F_{\alpha\beta}+F^{ext}_{\alpha}) = \sum_{\alpha}\sum_{\beta\neq\alpha}F_{\alpha\beta}+\sum_{\alpha}F^{ext}_{\alpha}$? | |
| Aug 24, 2012 at 11:49 | comment | added | Luboš Motl | Let me say the proof in words. The forces on each of 5 particles are either internal, from the other four; or the external, from someone else. Each of the forces may violate the momentum conservation i.e. change total momentum. However, the internal forces don't because they change the momentum of each of the 2 interacting particles oppositely, via action and reaction. So the only remaining term is the other one, from the external forces, and if the sum of these external forces vanishes, the total change of the momentum is zero. | |
| Aug 24, 2012 at 10:19 | review | Close votes | |||
| Sep 3, 2012 at 18:34 | |||||
| Aug 24, 2012 at 10:16 | comment | added | Luboš Motl | It's really hard to understand what you mean by it. This is a completely elementary straightforward derivation, there's nothing to add to it, and one may at most point out some particular step or something like that and ask about it. But if the problem is "derivation itself", it seems that your situation is hopeless because you effectively want the other users to teach you all of basic maths. | |
| Aug 24, 2012 at 9:43 | comment | added | alok | My actual problem is the derivation itself. | |
| Aug 24, 2012 at 9:23 | comment | added | Luboš Motl | The only thing you seem to be asking is why the second term in $\dot P$ was summed over $\alpha$. It was always summed over alpha, so the sum over alpha couldn't disappear. After all, $\dot P$ doesn't depend on any alpha index so it would be inconsistent if the right hand side did depend on it. It's very far to trace what's your actual problem. | |
| Aug 24, 2012 at 8:51 | history | asked | alok | CC BY-SA 3.0 |