Answer:
Momentum is a physical quantity equal to the product of mass of the object and its velocity. Since velocity is a vector (a quantity that has both magnitude and direction), momentum is a vector quantity as well. According to the law of conservation of momentum, the momentum is always conserved. This means that the initial momentum must be equal to the final momentum. Our initial momentum is zero since both, trolly and the girl on it are stationary. When the girl jumps, she has some velocity v. To compensate for that, the trolly will also move with some velocity v, but in the opposite direction so that the momentum stays conserved.
Step-by-step explanation: