ok, I think the answer to Paul's question regarding moving a door from rest against a door jamb is related to the property of the invisible hand. You see, since objects at rest will remain in rest unless acted upon by an outside force, we can assume that the object will remain. The other assumption that we make is that the other doors are also remaining at rest (given that the other doors remaining at rest would be the reasonable and prudent thing to do at the time.) Obviously they would have to weigh their own choices and determine which would bring them the most utility. If each of the doors was looking out for their own best interest, the invisible hand of the economy would come along and open Paul's aforementioned door with a force at least greater than mass times gravity (assume a graviational constant of 9.80 m/s^2) Mu is where we run into a problem. You see, Mu is different on each door that we find. My doors are seventy years old, they can kick stretch, and creak. The back door, in particular, takes something in the neighborhood of 8,000 N to open on a particularly cold morning. Whereas, the door to my house in Hope literally propels itself open (undoubtedly aided by Boyle's Law) I think we should assume mu is zero, and therefore I can kick the door open with much ease before declaring in a righteous voice that I despise physics with much gusto, and that I will be preparing a peanut butter and banana sandwich for my enjoyment.
Jeff C.
