What do you think accounts for the diffidence between the starting friction and the sliding friction

Once inertia is overcome and the object is in constant motion then sliding friction will mostly depend on the surface, environmental factors (wind, ice, etc...).  

example:  

To push a piano from 0 to 1m/minute you would need to over come a huge force pulling down the piano(gravity) but if it were already moving...to push it , it would require less force, if the environment (and the surface) was smooth and not uphill.

the mantle

explanation:

earth is divided into three layers: the crust or the outermost layer on which we live; the underlying, middle layer - the mantle - and the core (the innermost layer of earth).

aquifers are bounded above and below by impermeable rock. the driving force of groundwater movement in confined aquifers is pressure, rather than gravity.