The initial momentum of both of them is zero but when one friend a ball of 2kg to other Newton’s third law of motion comes into action. Therefore, when one friend throws the ball to other he exerts a force on the ball and receives an equal and opposite force. In order to conserve the momentum, the one who throws the ball will move backward changing his initial position.
Now the other friend catches the ball, Law of conservation of momentum comes into action. In this case the momentum of the ball before catching and after catching will be equal. Therefore, the second will experience a net force after catching the ball and therefore will move backwards in the direction of the force. Thus, the separation between them will increase.
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