Assume there is a boat of length 4 m weighing 40 kg. One man of weight 50 kg is at one end (A) and the other of weight 65 kg is at the other end (B). Both men now move towards the centre of the boat. Will the boat move in the water and if so by what distance ? (assume no friction between water & boat) Taking origin at A, COM originally is 22/15 from A. Since no external force acts on the system the COM remain stationary. Using this principle, assuming that boat moves by 'x' towards the left of the origin, it is obtained from the eqn for COM that 0 = 50(2-x) + 60(-2-x) + 40(-x) / (150) Solving this eqn gives x = 2/15 i.e. boat moves 2/15 m towards the left. THE DOUBT IS WHY DID THE BOAT MOVE WHEN NO EXTERNAL FORCE ACTED ON IT ?