A 48.2 kg girl is standing on a 129 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 1.66 m/s relative to the plank. What is her velocity relative to the ice surface?

Answer 1

Hint: the center of mass of the plank will stay in the same place as the girl walks along it.

Compute the speed of the plank using this fact. Then do a vector addition of the plank velocity and the girl's velocity relative to the plank (which was given)