Timeline for Why can we calculate force on a dipole from torque?
Current License: CC BY-SA 4.0
12 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Apr 6 at 17:25 | vote | accept | Ghorbalchov | ||
| Apr 4 at 15:23 | answer | added | Jerrold Franklin | timeline score: 0 | |
| Apr 3 at 19:22 | comment | added | hft | I think that you are correct about that | |
| Apr 3 at 18:52 | comment | added | Ghorbalchov | Yes I agree. So the fact that $U_\mathrm{mech}$ as derived from angular displacement alone happens to also give the force (i.e. there is no contribution from only the dipole position) is just lucky to this situation, and wouldn't be save to assume I'm general? | |
| Apr 3 at 18:47 | comment | added | hft | ... $F_i = -\frac{\partial U_{mech}}{\partial q_i}$ where $q_i$ could be $\theta$ or $q_i$ could be other coordinates. | |
| Apr 3 at 18:45 | comment | added | hft | I think it is probably better to be more careful like Feynman. I have not looked at it carefully, but I think that the first argument about the torque helps us see that the generalized force associated with the angular displacement (with all other degrees of freedom held fixed) can be derived from $U(\theta) = -\mu B \cos(\theta)$. But then Feynman seems to want to generalize to other generalized coordinates and asks, say, what if there is a translation in space of the dipole, say, in a non-constant field. So, he seems to give the argument to show that, more generally... | |
| Apr 3 at 18:42 | comment | added | hft | Hard to say why your lecturer did what they did. | |
| Apr 3 at 18:40 | comment | added | hft | If you already know the answer then it is safe to assume so... but I think he is trying to explain why this is safe... This seems to be what he is saying in the two sentences surrounding his Eq. 15.11. | |
| Apr 3 at 18:40 | comment | added | Ghorbalchov | I am asking because my lecturer did just that; derived the force from the torque without further elaboration like Feynman. | |
| Apr 3 at 18:37 | comment | added | Ghorbalchov | But in other words he had to check explicitly that $U_\mathrm{mech}$ was also the work done in bringing in the dipole? It wouldn't be safe to just assume this to be so? | |
| Apr 3 at 18:35 | comment | added | hft | "Is there some trick I am missing to explain..." Isn't this explained in the paragraphs following his statement about virtual work and forces? I.e., the paragraphs starting off with: "We can show for our rectangular loop that $U_{mech}$..." | |
| Apr 3 at 18:12 | history | asked | Ghorbalchov | CC BY-SA 4.0 |