Calculating Impulse Ratio for Colliding Molecules A and B

[Fuzzykatecake]

Two identical molecules A and B each of mass M travel with the same velocity towards a vertical wall at an angel 20 degrees with respect to the normal to the wall. A rebounds from the wall with the same speed while B sticks to the wall.

Determine the ratio of the implus experience by A to that by B during collision.

I know the inpluse experienced by A is
final momentum - initial momentum=
Mvcos20-(-mvcos20)

But for B, why is the impluse mv? And not 0- mvcos20

 

[Orodruin]

What is the velocity of B before the collision? What is the velocity after? As a result, what is the difference in B's momentum before and after?

 

[Fuzzykatecake]

What is the velocity of B before the collision? What is the velocity after? As a result, what is the difference in B's momentum before and after?

Isn't the velocity of B before collision vcos20. And velocity after is 0 since is stick to wall. Therefore the momentum is 0-mvcos20.
But the correct answer is mv which I don't understand why

 

[Orodruin]

Isn't the velocity of B before collision vcos20.

No, that is the velocity component orthogonal to the wall.

 

[Fuzzykatecake]

No, that is the velocity component orthogonal to the wall.

But why for A is vcos20, what's the difference?

 

[Orodruin]

But why for A is vcos20, what's the difference?

A does not stick to the wall. The momentum component of A orthogonal to the wall is simply reflected while that perpendicular to it is unchanged.