A 44 kg girl is standing on a plank that has a mass of 170 kg. The plank, originally at rest, is free to slide on a frozen lake, which is a flat, frictionless supporting surface. The girl begins to walk along the plank at a constant speed of 1.37 m/s relative to the plank. What is her speed relative to the ice surface? My attempt: The horizontal force the girl exerts on the plank is f=1.37(44)=60.28N so the speed of the plank relative to the ice should be 60.28=170a a=.354ms and her speed relative to the ice should be 1.37-.354=1.016 but this is incorrect, what am i doing wrong?