If the snowball is just rolling across grass or concrete, it loses kinetic energy
and slows down because of friction with whatever it's rolling through.
If it's also picking up more snow or slush as it rolls, then it has to slow down
even more. There's no way for it to get more kinetic energy, unless it has
rocket engines on it. So if it maintains the same kinetic energy but its mass
keeps growing, then it has to slow down.
