Discussion

The force between surfaces in contact that resists their relative tangential motion (slipping).

Types: static & kinetic

Classical Approximations

• independent of
  • surface area,
  • speed (except when v = 0), and
  • temperature
• depends on the nature of the surfaces in contact and is
• directly proportional to the normal force.
  • Interesting quote …
    Guillaume Amontons (1663-1705) France
    It was Guillaume Amontons who first established that there existed a proportional relationship between friction force and the mutual pressure (or force) between the bodies in contact. We recognize that relationship when we divide friction force by normal force - and identifying the quotient as the "coefficient of friction". Amontons' paper "De la résistance causée dans les machines" was published in 1699 in Memoires de l'Académie des Sciences.

Æ’ = μN

microscopic description

miscellaneous stuff

• Recently it has been shown that the lubricant properties of graphite disappear under ultra high vacuum, and hence that molecules of gases, such as oxygen and nitrogen, most probably act as a kind of molecular grease to help the sheets slide past each other.
• Roughness is a minor factor affecting friction. Friction is often higher between smooth surfaces. Insects can walk on windows.
• If friction is independent of surface roughness, why do tires have tread? Tire tread sheds water.
• Teflon has such a low coefficient of friction that it often peels off of pots and pans. (Use wooden or plastic utensils.) How do you get it to stick? Dreadlocks analogy: Teflon is a polymer, individual strands of hair are slippery, but strands can tangle to the point where they can't be separated.
• Humans have very little body hair. Why are certain areas still densely covered with hair? Evolutionary advantages. Describe them!
• Dynamic friction even exist on the galactic scale. The gravitational tug of passing planets is much the same as the electrostatic forces between passing atoms. The coherent motion of groups of planets will eventually degrade into the random motion of individual planets.

Coefficients of Friction for Selected Interfaces (in order of generally decreasing value)
μs	μk	interface
	1.16	rubber	–	rubber
	1.02	rubber	–	concrete
	0.72	car tire	–	asphalt
	0.35	car tire	–	grass
0.8–1.0		skin	–	metals
0.9–1.0		glass	–	glass
	0.9	sheep	–	steel mesh
	0.7	sheep	–	plastic batten (⊥)
	0.6	sheep	–	plastic batten (∥)
	0.6	sheep	–	wood batten (⊥)
	0.5	sheep	–	wood batten (∥)
0.58		steel	–	steel
0.4		brakes	–	cast iron
0.6		wood	–	brick
0.2–0.6		wood	–	metals
0.29	0.22	wood	–	felt
0.28	0.17	wood	–	wood
0.3		snow	–	nylon
0.04–0.4	0.04–0.4	snow	–	hickory, waxed
0.1		graphite	–	graphite
0.1		graphite	–	steel
	0.03	ice	–	steel
0.05–0.5	0.02–0.09	ice	–	ice
0.2		teflon	–	steel
0.04		teflon	–	teflon
	0.0044–0.0057	ankle cartillage	–	synovial fluid
	0.0013	tendon	–	sheath