Consider a gas enclosed in a box. A molecule of mass m, having a velocity -2i+3j+4k collides with a wall parallel to the xz plane. What will be its velocity after collision and its change in momentum? i,j & k are unit vectors along the x,y & z axis.

Correct Answer: -2i-3j+4k, -6mj
On collision with a wall parallel to the xz plane, only the y-component will change. Thus, new velocity will be -2i-3j+4k. The change in momentum will be -3mj-3mj = -6mj.