I find that the forces acting on a pendulum bob are treated in two different (contradictory?) ways. Let the angle that the cord makes with the vertical be x, the mass of the bob m and the tension in the cord T. 1. In one case, the weight of the bob is resolved into components so that mgcos(x) is along T, so T = mgcos(x). 2. In another case, the tension is resolved into ITS components, so that mg = Tcos(x). Obviously only one of them can be correct at one time. I can't figure out which of them to use and when. I am a physics teacher, by the way. Thanks in advance.