Simple wuestion about the sign(+/-) in a momentum conservation problem

[vande060]

Homework Statement

http://s861.photobucket.com/albums/ab174/alkaline262/?action=view&current=momentum.jpg

why is it + w instead of -w, it seems like it sould be negative because the recoil velocity is in the negative x direction

you have to admit that is a ms paint masterpiece :D

Homework Equations

The Attempt at a Solution

 

[quanticism]

Yeah, if w is a positive number, then it should be negative.

They might have been treating w as a vector (in this case, you would use +w since the diagram shows w pointing in the negative x direction) but then that equation isn't really consistent since they're not all expressed as vectors.