wa  =  0.5;
wb  =  4.0;
wc  =  2;
ContourPlot[((thetaDot/wc)^2 - (wa/wc)^2*Sin[theta]^2)/
            2 - Cos[theta], 
{theta, -Pi, 
          Pi}, {thetaDot, 0, 10}, 
        FrameLabel  ->  {"θ (rad)", "dθ/dt (rad/s)\
"}, 
RotateLabel -> 
          False];
ContourPlot[((thetaDot/wc)^2 - (wb/wc)^2*Sin[theta]^2)/
            2 - Cos[theta], 
{theta, -Pi, 
          Pi}, {thetaDot, 0, 
          10}, 
FrameLabel  ->  {"θ \
(rad)", "dθ/dt (rad/s)"}, 
RotateLabel -> 
          False];