Respuesta :
Answer:
When the boy has first started the motion this is similar to what kangaroo moves.
Initially the boy was standing and only force of gravity due to his own mass was acting.
When the boy started leaping an acceleration happened that caused his motion. His leg were bent to push above ground to leap, this provided an acceleration to the center of mass of the boy. and equal and opposite force was acted on the boys feet's touching the ground.
So according to Newton's third law action force was acted on the ground by the boy feet, and the reaction force was the opposite force by the rock and this caused the boy to accelerate.
