Gravitational force from a ring on a point mass at distance x

[carlosbgois]

Homework Statement

What is the intensity of the force $F_{g}$ between the ring and a mass 'm', which is at a distance 'x' from the center of the ring?

http://img52.imageshack.us/img52/6859/ringre.th.jpg

Uploaded with ImageShack.us

The Attempt at a Solution

I have got to my own answer, but it is different from the given one. Here's what I did:

There's a potential energy between 'm' and a dM from the ring, which is given by $dU=-G*m*dM/d$ and $d=\sqrt{r^{2}+x^{2}}$.

From this, I can find the total potential energy by integrating dU from 0 to M, which gives me $U=-\int^{M}_{0}\frac{G*m*dM}{d}=\frac{-G*m*M}{\sqrt{r^{2}+x^{2}}}$

As the variation in the potential energy is equal to the negative of the work done, I did $-\int^{0}_{d}F(d)*dd=\frac{-G*m*M}{\sqrt{r^{2}+x^{2}}}$$\Rightarrow$$F(d)=\frac{2*G*m*M}{(r^{2}+x^{2})^{3/2}}$

So, what's wrong? Thanks

 

[carlosbgois]

I think $-\int^{0}_{d}F(d)*dd$ should be [itex]$-\int^{0}_{d}F(d)*cos\theta*dd$, but even this way I didn't get the right answer.[/itex]

 

[carlosbgois]

Sorry, forgot to post the given answer for the exercise: $F = \frac{GMmx}{(r^2+x^2)^{3/2}}$