  We propose a simple mechanical model of locomotion induced by internal force
and friction. We first construct a system of two elements as an analog of the
bipedal motion. The internal force does not induce a directional motion by
itself because of the action-reaction law, but a directional motion becomes
possible by the control of the frictional force. The efficiency of these model
systems is studied using an analogy to the heat engine. As a modified version
of the two-elements model, we construct a model which exhibits a bipedal motion
similar to kinesin's motion of molecular motor. Next, we propose a linear chain
model and a ladder model as an extension of the original two-element model,. We
find a transition from a straight to a snake-like motion in a ladder model by
changing the strength of the internal force.
