Re: What makes a heavier object going down a hill get there first?

If the surface of the hill had no friction then both balls should arrive at 
the same time. Some force of friction is what slows it down. If the heavier 
object goes down the hill faster, then it must have less friction retarding 
its motion. The lighter object may have a rougher surface, or some other 
properties that's making it slower. Friction is what makes the difference.

[note added by MadSci Admin:  The objects may also have different relative
moments of inertia, which depend on the distribution of mass in the 
objects and affect the rotation of the objects.]