Q:

Understanding a formula on acceleration

We are given the formula in vectors, $\underline{a}=(μ\underline{r} \times \underline{v})/r^3$ where "$\underline{a}$" is the acceleration of any particle, "$\underline{r}$" is the displacement and "$\underline{v}$" is the velocity and "$μ$" hasn't defined and "$\times$" is the cross product in vectors. Then we are supposed to proceed the lesson. I want to know 
how this formula developed.
Can somebody explain this please?

A:

Your question is not very clear as to where this equation comes from. Is it just an example problem and you are wondering what would produce such a physics?
( One circumstance where a force like this could occur is in the case of a particle moving in a magnetic field. In particular, consider a "magnetic charge" at the origin producing a field $ B \propto \frac{\hat{r}}{r^2} $, then the force $ F = q v \times B $ will give a formula similar to the problem with the appropriate constant $ \mu $. )

