1. The problem statement, all variables and given/known data A skateboarder on a horizontal surface can accelerate from rest just by moving the body, without touching external support. Investigate the parameters that affect the motion of a skateboard propelled by this method. 2. The attempt at a solution I think that "friction" and "changing the center of mass" are what cause this happen. But I got some problems in how to explain the parameters that affect the motion exactly. And as I remebered, the center of the mass won't change. But what I tried to make to skateboard move is to kneel a little bit and make the upper part of my body forward, and then move the skateboard forward by my waist (and the movement shall be faster than the one I move my upper body).