Is it really possible for a system to decrease its velocity with no forces acting on it, just because the mass in it is "varying"? Consider for example a freight car and a hooper from which sand is released in to the car. The freight car will decrease its initial velocity if there is no force supplied, but that's not because its mass is increasing, but because the sand that comes in to the freight car produces friction with it (equivalently "tries" to be accelerated) and the friction force is the one that makes the velocity of the freight car decrease. So actually there is a force, which cause the acceleration (deceleration). Is this correct? Or it is really just the increasing of mass that change the velocity of the freight car?