The motions of hammer were analyzed to understand the mechanism of acceleration using variable-length pendulum and hula-hoop model using energy pumping mechanism. The condition that makes the time derivative of the energy positive is derived as energy pumping for hammer. The condition is expressed in terms of tugging force times velocity to pump hammer energy and tangential acceleration on the handle. In this study, motions of hammer were analyzed and numerical experiments were performed to examine the validity of the theory.