The compound pendulum.
Obtain the equation of motion [17], page 228, for a compound pendulum.
method1.
suppose that the vertical plane of vibration of the pendulum is chosen as the xy plane [Fig. 9-24] where the z axis through origin o is the horizontal axis of suspension.
Let point C have the position vector a relative to O. Since the body is rigid.
is constant and is the distance from O to C.
the only external force acting on the body is its weight mg=-mgj acting vertically downward. Thus we have.
total external torque about z axis.
where k is a unit vector in the positive z direction [out of the plane of the paper toward the reader].
also, the instantaneous angular velocity is
so that if m is the moment of inertia about the z axis
angular momentum about z axis