Given:
The mass of the small block is m.
Initial speed of this block is v.
The mass of the bigger block is M.
Initial speed of this block is zero.
At point A, as the small block transfers its momentum to the bigger block, both the blocks move with a common velocity V (say) in the same direction as v.
Using the law of conservation of linear momentum, we can write:
Initial momentum = final momentum
Therefore, the speed of the bigger block when the smaller block reaches point A of the surface is.