Question
A man of mass 60 kg is standing on a boat of mass 140 kg , which is at rest in still water. The man is initially at 20 m from the shore. He starts walking on the boat for 4s with constant speed 1.5 m/s towards the shore. The final distance of the man from the shore is
Joshi sir comment
distance covered in boat = 1.5*4 = 6 m
and for making the position of centre of mass undistubed displacement of boat in opposite direction = x (let)
by law of conservation of momentum (140+60)*v = 60*1.5
so v = 90/200 = 0.45 m/s
so displacement of boat = 0.45*4 = 1.8 m
so final distance of the man from the shore = 20-6+1.8 = 15.8 m