Why do masses cancel out when calculating y-components of momentum?

[oridniv]

Why is it that in many momentum questions, the mass gets canceled out when calculating y-components of momentum? All it says in my textbook is that they cancell out since the colliding objects are going in opposite directions, but this doesn't make any sense to me. Can someone please help?

 

[Cyrus]

Can you type exactly what your book says. I think you might be misreading it.

 

[oridniv]

nm, i misread something, i figured it out though so don't worry