I have an ideal pendulum. In (S^1 x R):

differential of theta = p
differential of p = -sin(theta)

this is all the information i have. i am trying to understand the system. are these polar co-ordinates with p being the displacement from origin and theta the angle? I am trying to understand the system in order to draw a phase portrait. At the moment I am completely stuck. Any help, tips, pointers all much appreciated.
Thanks in advance..