I have done a bit of work and am not seeing where I am going wrong in this. So here's a try: A 46.8 kg girl is standing on a 130 kg plank. The plank, originally at rest, is free to slide on a frozen lake, which is a flat, frictionless surface. The girl begins to walk along the plank at a constant velocity of 1.75 m/s relative to the pank. a)what is her velocity relative to the ice surface? b)what is the velocity of the plank relative to the ice surface. What I've done is find the power of the girl (46.8)(1.75) to get 81.9. Then to find the velocity of the plank relative to the ice, take that power and divide by the mass of the plank so 81.9/130=.63 m/s. Then the velocity of the girl on the plank minus the plank on the ice: 1.75-.63=1.12 Problem is..that's not right. Any help would be appreciated!