If you said someone had a velocity of -12mph and they were traveling north? Wouldn't it mean that they were traveling 12mph south?
This is a quote from here:
if something [object-x] moving to the right was taken to have positive momentum, then one should consider something [object-y] moving to the left to have negative momentum.
But isn't Momentum a vector, so the direction should be specified separately to the number. What I mean is, object-y with -1200kg m/s and object-x with 1200kg m/s should both have momentum in the same direction. But this can't be because as the quote says they are moving in opposite direction.