While two forces act on it, a particle is to move at the constant velocity v = (3 m/s) i - (4 m/s) j. One of the forces is F1 = (4 N) i + (- 9 N) j. What is the other force? 2. Relevant equations: Unsure 3. The attempt at a solution - I know this is an easy problem, but I have one problem with it. I'm not sure how to relate force and velocity. So I'm not even sure where to start. Any hints to point me in the right direction would be great.