Originally Posted by
asgelle
We're not looking at the effect. but the change in the effect with changes in mass. A 20% change in mass increases the driving force from gravity by 20% and also increases the ******ing force from rolling resistance by 20%. Since the OP wants to neglect this change, I'd say it's incumbent on him to show it's negligible. From the above, it's not obvious that it must be.
It is a reasonable assumption that frictional forces contribute a minor fraction of the ******ing forces as compared to air drag.