Question

the equation to find the momentum of a particle

Answer 1

\[p=mv\]

where p is momentum and m is the mass of the particle and v is velocity. Momentum is a vector quantity.

Answer 2

if it is linear momentum then \[\bar{p}=m\bar{v}\], where the bars denote vector quantities. If you mean angular momentum then it is the product of the linear momentum and the radius of curvature i.e. \[\bar{L}=m\bar{v}\bar{r}\]

strictly speaking, in vector notation it angular momentum is the cross product of the radial vector and the linear momentum (both of which are vectors) i.e.\[\bar{L}=\bar{r}\times\bar{p}\], the x denoting cross product.