And on this elementary surface, let me say that velocity of the fluid particle is q be the velocity of the particle. And let me say that n is the out at the normal on the surface, then in that case then and let me say rho is the density of the fluid density of the fluid. So, if rho is the density, then we have already had defined n at q bar, s is the elementary mass. Then what will happen mass of fluid that will be what will happen to the mass of fluid That will enter per unit time, through this cross section through the surface element through d s. The amount of fluid that will enter through this surface elementary d s, will be minus rho q bar into dot n hat d s in outer direction it will enter through the fluid. And then already I have mentioned that here rho is the density q is the velocity, then what will happen to the total mass of the fluid.