A boat is likely to capsize if the persons in the boat stand up. Why ?

A boy having a mass of 40kg stands at one end A of a boat of length 2m at rest. The boy walks to the other end B of the boat and stops. What is the distance moved by the boat? Friciton exists between the feet of the boy and the surface of the boat. But the friction between the boat and the water surface may be neglected. Mass of the boat is 15kg .

Boat is a means of

A person P of mass 50 kg stands at the middle of a boat of mass 100 kg moving at a constant velocity 10 m/s with no friction between water and boat and also the engine of the boat is shut off. With what velocity (relative to the boat surface) should the person move so that the boat comes to rest. Neglect friction between water and boat.