Physics education research, electronic materials, and the musings of Christopher Moore, Ph.D.

A 5200—kg open railroad car rolls along with a constant speed
of 7.80 m/s on a level track.  Snow begins to fall vertically and       fill the railroad car at a rate of 71.7 g/s.  Assuming the frictional
    forces between the wheels of the railroad car and the track are
negligibly small, find the speed, in m/s, of the railroad car 30.0 min after the snow begins to fall and then determine whether conservation of mechanical energy holds true.