Momentum thickness of boundary layer

Correction: 7:43 and ahead , kindly read u/U as U/u, Its a typo error.
When a fluid flows over some surface and boundary layer is formed, then the velocity of fluid inside the boundary layer becomes less than the free stream velocity. This lead to loss of momentum within the boundary layer. To compensate the momentum loss at a particular point of surface , we can increase the vertical height of free stream velocity up to some level called as momentum thickness. This lecture analyzes the same and derivation of momentum thickness is narrated...