Answer: the horizontal component of total momentum
Explanation:
Since the open cart is rolling to the left on the horizontal surface, the quantity that has the same value just before and just after the package lands in the cart is the horizontal component of total momentum.
Momentum, is the product of the mass of a particle and the velocity of the particle. The change of momentum depends on the force which acts on it. The addition of the the individual momenta is the total momentum.
