An open freight car rolls friction-free along a horizontal track in a pouring rain that falls vertically. As water accumulates in the car, its speed

Question

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As water accumulates in the car, its mass increases. According to the principle of conservation of momentum, if no external horizontal forces act on the system (car + water), the total momentum of the system must remain constant. Since momentum is the product of mass and velocity, an increase in the mass of the car (due to accumulating water) must result in a decrease in its velocity to keep the momentum constant.

An Open Freight Car Rolls Friction-Free Along a Horizontal Track