A girl of mass M_G is standing on a plank of mass M_P. Both are originally at rest on a frozen lake that constitutes a frictionless, flat surface. The girl begins to walk along the plank at a constant velocity V_GP to the right relative to the plank. (The subscript GP denotes the girl relative to the plank.) (a) What is the velocity V_PI of the plank relative to the surface of the ice? (Use the following as necessary: V_GP, M_G, and M_P. Indicate the direction with the sign of your answer. Let the positive direction be in the direction that the girl walks.)