A girl of mass mG is standing on a plank of mass mP. 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 υGP to the right relative to the plank. (The subscript GP denotes the girl relative to plank.) (a) What is the velocity υPI of the plank relative to the surface of the ice? (b) What is the girl’s velocity υGI relative to the ice surface?