Force Density in Kangaroo

I have a planar mesh in which each edge has a different force density associated with it. I want to apply gravity loads to the mesh using Kangaroo. I know by setting the target length of the Edge Length to zero I can do force density. Will setting the weight of the edge length to the force density ensure the force density of the edges remains constant? When I try to do this the component gets very unhappy with me and doesn’t let me do it!

What do you mean ‘gets very unhappy’?
Yes, a length goal with a rest length of zero gives a constant force density, but a bit more context about what you are trying to do and what about it currently isn’t working would be needed to be able to help you.

I have a planar mesh which I have already found and each edge has an associated force density. I then want to apply gravity loads to get a 3D net. To do this, I need to input the force density into the Kangaroo solver. When I take the edge length component, I first attach the list of force densities as the strength. I then attach the mesh. When I do this, Grasshopper becomes unresponsive and eventually disconnects the mesh.

If you want to set separate strengths for each edge, do not use the EdgeLength component (which only takes a single strength value as input), but instead extract the edges and use a Length goal component, with a list of edge lines as input and a matching list of strengths.
Sounds like you are currently duplicating the entire system for every edge.

Why not just compute final lengths multiply by weightxlenght divide by 2 and apply these as z forces on the nodes ?

Force densities are related to equilibrium of the pre-stressed net , if you want gravity effect you.must apply masses at the nodes