I was wondering what rolls down a ramp faster - a tube (capped) full of dirt of a tube full with water. So i tried it and found that the water rolled faster -when it's mass was less then dirts mass and also when it was more. i thought of two reasons for this: 1) the water doesn't spin around in the tube as much so less energy is wasted on rotation. 2)the water is more uniform so it rolls better. what's the real reason? Thanks.