Vector subtraction is needed if you want to find change in a vector, be it velocity, momentum, etc
eg. change in velocity = final velocity - initial velocity
change in momentum = final momentum - initial momentum
But subtraction is same as addition of a negative vector
eg C = B - A = B + (-A)
to do subtraction , just add B to A (in the opp direction)
But be careful as A-B is not the same as B-A. Hence final vector - initial vector has to be carefully done. Start with final vector and continue with the initial vector in the opposite direction.View diagrams to confirm your understanding.
