Hi guys. I hope this is the right thread for my question. The question is for my project and I can assure you it's not my homework. I know the solution to my problem isn't that complicated but I just lost my self with the problem and I don't have any one to talk with about my problem.(adsbygoogle = window.adsbygoogle || []).push({});

Anyhow, here is my problem. I need to find the forces between two rods. Each rod is constructed from 3 particles (O--O--O) and the potential between the rod is given by:

[tex]U = f(\bar r) \cdot g(\bar u_1 - \bar u_2) [/tex].

Where

[tex] \bar r = \bar r_{22} - r_{21} [/tex]

That is, [tex] r [/tex] is the distance between the two middle particles. [tex] \bar u_i [/tex] is give by

[tex] \bar u_i = \bar r_{i3} - \bar r_{i1} [/tex]

which means it is the vector that shows the direction of the rod. I need to find forces between the particles:

[tex] F = \frac{dU}{dr} [/tex].

My problem is with the derivative of [tex] \bar u_1 - \bar u_2 [/tex] with respect to [tex] \bar r [/tex]. I tried several ways but I alway end up with [tex] \frac{du_1}{d \bar r} - \frac{du_2}{d \bar r} [/tex].

What have I missed?

Thanks.

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# Expressing start of a vector using a point on the vector

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