A smooth hemisphere ‘S’ and a particle ’P’ are of same mass ‘m’. P is dropped from a height h as shown in figure. A hemisphere ‘S’ is kept on a smooth horizontal surface. The particle ‘P’ collides elastically with hemisphere at the point shown in the figure. If $\theta ={45}^o$ and after collision the velocity of the particle becomes horizontal, then  the velocity of hemisphere ‘S’ after collision in $m{s}^{-1}$ is (h=20m) $(g=10m{s}^{-2})$