A man of mass 45 kg. is standing on a uniform rolling board of length 10 m. and mass (mb = 30 kg). If the man starts to move from one end to move to the other end of the board,
a. When the man reaches the other end, how far does the board retreat?
b. If the man moves with a constant speed of 2 m/s relative to the board, how fast does the board retreat?