THE FREE HANGING PENDULUM
Coordinates of point cg in terms of point p, angle
, and length 
Θ(t) is a function of time, so we use the Chain Rule to differentiate sin(Θ(t)).
(because the pivot point is stationary. Velocity = 0)
Θ(t) is a function of time, so we use the Chain Rule to differentiate cos(Θ(t)).
(because the pivot point is stationary. Velocity = 0)
