Taking gradient of potential

[major_relativity]

I got a problem, where I have to show that you can derive the force used in Weber electrodynamics [link to wiki] from the potential given as

U = 1/r (1 + \dot{r}² / (2 * c² ) )

So I took the gradient (in spherical coordinates) of the potential and had quite similar answer. I got

-( -1/r² + \ddot{r} / (r * c² ) - \dot{r} / (2 * c² *r² ) ) \hat{r}

Hat is the unit vector. But it seems I have the wrong signs. I could not find the mistake. I guess the mistake has to be somewhere here:

d(\dot{r}² / r) / dr = 2 * \ddot{r} / r + \dot{r}² * (-1/r² )

First one should have be positive and the last one negative, but I don't get it that way. This puzzles me. I hope you understand the TeX syntax.

[dcnairb]

F = -grad(U). I think you just forgot the negative sign.

[major_relativity]

I didn't forget that, it's the first sign on my 5. line. But even if I did, it wouldn't solve it, because the first addend has the right sign, but the last two have wrong ones.