# Homework Help: System of forces in equilibrium

1. Nov 5, 2008

### vp_

A force $F_1=i-3j-2k$ at the point $-2i+9j$, another force $F_2=2i+j-3k$ at the point $-i+yj-k$ and a third force $F_3$ are equivalent to zero. Find $y$ for this to be possible. Find $F_3$ and its line of action in this case

I infer that "equivalent to zero" means this system of forces is in equilibrium. I understand that for equilibrium, the resultant force on the system has to be zero, and the sum of moments must also be zero.

I understand how to calculate moments ($M=r \times F$), but fail to see how I can deduce $y$ without any information on where $F_3$ acts. I have $F_3=-3i+2j+5k$ by working out $F_1+F_2+F_3=0$ but am stuck beyond that.

Any help would be appreciated.