The problem is shown in the photo above. I would like to discuss part B.
conservation of linear momentum
The Attempt at a Solution
the solutions say that we should have the following approach:
Mb*vb = [Mb + Ms] * vFinal.
Plug in the numbers and get vF = 0.57 m/s
However, I do not understand this. I thought that momentum is a vector and that while the block is moving forward, the slab is moving backward, so I would need to have the following:
Mb*vb = Mb*vF + (Ms* -vF)
notice the negative vF for the final momentum term of the slab. I put a negative because the slab would be moving backwards with a negative velocity.
Why is my approach not correct? I got a different vF than 0.57.