I am looking for an easy way to explain to my 2nd grade son which ball will roll down a ramp faster. I have a ramp (more like a trough) set at about 30 deg angle incline. He is going to roll a baseball, pool ball, golf ball, and basket ball down the rramp. He is only 2nd grade so I can't get into the math since he hasn't had multiplication yet. Any suggestions? - This is his science fair project.