# Relative motions

1. Sep 18, 2005

### Sarahchichi

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

2. Sep 18, 2005

### Integral

Staff Emeritus
Well, if you need the sum of 2 vectors you should add, if you need the difference you should subtract.

Your question is to general, you need to ask a specific question.

3. Sep 18, 2005

### Dr.Brain

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.

BJ