Fig 6A lamp on two cablesthe knot is in equilibriumΣF = 0 at the knot
A rope can only pull, and only along itself. That pull is tension. Take the knot as your object. Three forces act on it: the two cable tensions and the lamp's weight. It isn't moving, so they add to zero, which gives two equations (x and y) for the two unknown tensions. The shallower a cable, the harder it has to pull: a cable nearly horizontal needs a huge tension to hold up anything at all. That's why power lines sag.