Suppose an unpowered but moving railroad car receives some coal (basically at rest) from a stationary conveyor belt. This coal therefore only carries chemical energy into the car. This mass transfer will

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When coal is transferred to the moving railroad car, the total mass of the car increases. According to the principle of conservation of momentum, if no external force acts on the system, the total momentum of the system (car plus coal) before and after the coal transfer remains constant. Since momentum is the product of mass and velocity, an increase in the mass (due to the addition of coal) without an increase in the system's total momentum means the velocity (speed) of the car must decrease.

Suppose an Unpowered but Moving Railroad Car Receives Some Coal