The picture on the wall
A framed picture hanging on a wall stays still. But here the three forces are not parallel — and that is what makes this case different from the ones before it.
Figure to be added
The picture stays still because the tensions acting along the two cords (F1 and F2) and the weight of the picture (W) — the three of them — are in equilibrium.
Coplanar, but not parallel
These three forces lie in one plane, but they are not parallel to one another as they were in every case described before. The two cords slope, so the forces along them slope too.
So do the conditions change?
The first of the three conditions we found for parallel forces is satisfied here as well — the three forces lie in one plane. But when they are not parallel, is there something new to say about their lines of action? That is what the next activity sets out to find.
Let us now look for the conditions a body needs in order to be in equilibrium under three coplanar but non-parallel forces like these.