  Taking all the magnon modes into account, we derive the skyrmion dynamics in
response to a weak external drive. A skyrmion has rotational symmetry and the
magnon modes can be characterized by an angular momentum. For a weak distortion
of a skyrmion, only the magnon modes with an angular momentum $|m|=1$ govern
the dynamics of skyrmion topological center. The skyrmion inertia is determined
by the magnon modes in the continuum spectrum. For a skyrmion driven by a
magnetic field gradient or by a spin transfer torque generated by a current,
the dynamical response is practically instantaneous. This justifies the rigid
skyrmion approximation used in Thiele's collective coordinate approach. For a
skyrmion driven by a spin Hall torque, the torque couples to the skyrmion
motion through the magnons in the continuum and damping, therefore the skyrmion
dynamics shows sizable inertia in this case. The trajectory of a skyrmion is an
ellipse for an ac drive of spin Hall torque.
