It appears there's something missing. Either we need to know the coefficient of friction or the velocity of the trolley when empty (I think). Look at it this way. The size of the load to give you a constant velocity up the ramp would have to be lower if the friction was higher (with a different velocity). If the friction is high enough the trolley will not move at all even with no load. Perhaps I'm missing something though - not sure as my mechanical engineering skills are very rusty. I'll give it some more thought later.
