Need help on this review question for exam A stream of gas c

A stream of gas consists of n molecules. Each molecule has mass m and speed v. The stream is reflected elastically from a rigid surface as shown. The magnitude of the change in the total momentum of the stream is:

Solution

Let momentum be M = Mx + My , sum of two vectors as moment itself is a vector

Momentum of air particle Mx1 = m v cos 30 , My1 = mv sin 30,

Now after the particle reflects Mx2 = m v cos 30 , My2 = - mv sin 30

So change in momentum = Mx1 - Mx2 + My1 - My2 = 2 mv sin 30 = mv