# Homework Help: What additional force is needed to keep the object at rest?

1. Sep 14, 2011

This is a problem from my Calculus class in the sections about vectors.

1. The problem statement, all variables and given/known data
Two forces F1 = 2i + 6j and F2 = 2i -2j act on an object. What additional force is needed to keep the object at rest?

2. Relevant equations
I'm not sure where to start.

3. The attempt at a solution
I know for the body to be at rest it has to equal 0.
I've tried finding the displacement between the two given vectors which gives me <0, 81>. I'm not sure if that is even relevant. The only thing I really get right now is that something has to equal zero. I feel like this is an easy problem but I don't know where to get started..

2. Sep 14, 2011

### nonequilibrium

So we're looking for the third force F3, correct? Let's write it out as F3 = x i + y j + z k where x, y and z are unknown at this moment. (I'm assuming you're working in three dimensions, if you're only working in two, forget the "z k" term)

Now think about what the 'only thing you really get right now' tells you about the relation between these three forces...

3. Sep 14, 2011