Simple Nature - Light and Matter

i/Static friction: the tray doesn’t

slip on the waiter’s fingers.

j/Kinetic friction: the car skids.

come unstuck, and the coin begins to slide over the paper. Kinetic

friction slows down this slipping motion significantly. In terms of en-

ergy, kinetic friction is converting mechanical energy into heat, just

like when you rub your hands together to keep warm. One model

of kinetic friction is that the tiny irregularities in the two surfaces

bump against each other, causing vibrations whose energy rapidly

converts to heat and sound — you can hear this sound if you rub

your fingers together near your ear.

Fordrysurfaces, experiments show that the following equations

usually work fairly well:

Fs,max≈μsFn,

and

Fk≈μkFn,

whereμs, the coefficient of static friction, andμk, the coefficient of
kinetic friction, are constants that depend on the properties of the
two surfaces, such as what they’re made of and how rough they are.
self-check D

1. When a baseball player slides in to a base, is the friction static, or
   kinetic?


2. A mattress stays on the roof of a slowly accelerating car. Is the
   friction static, or kinetic?


3. Does static friction create heat? Kinetic friction? .Answer, p.
   1055

Section 3.2 Force in one dimension 157