Carlos B. answered • 03/14/19

Bacherlor of science in physics

Think of yourself being in free space holding a box. At first time you and the box have no velocity so the total momentum is zero. Then you decide to throw the box so you apply a force on in it, and the box starts to move and getting away from you. Now the box has some momentum different from zero, but since the momentum of the whole system (you and the box) must remain constant, a neggative momentum must balance the possitive momentum of the box. This means that you applied a force to the box, but in the same time, the box applied a force to you in the opposite direction, balancing the total forces of the system (whose sum have to be zero) and giving you negative momentum, so now you are moving in the opposite direcction of the box. This is how rockets work, they launch mass downwards, and at the same time that mass launch the rocket upwards.