# Relative motions

hello, everybody.
I have a question about relative motion. I don't know how to determine whether it is sum of vectors or difference of vectors.
Vab=Va-Vb
Could you explain to me? When should use the sum of vectors and when should use the difference of vectors?

From
Sarahchichi

Integral
Staff Emeritus
There's no strict rule . Take one direction to be positive , and Apply $V_ab = V_a - V_b$ . Take V to be positive if it is in the direction which u took to be positive or take it as negative if it is in direction opposite to the one you took as positive. The final relative velocity will be positive or negative and thus will telll you , in which direction it is directed.