Transcript Coefficient of Friction

What do all of these things have in common?
4.2 - Friction
Friction:
• Resists motion
• Acts in direction opposite to direction of
motion
• Caused by electrical forces between contact
surfaces
What are some everyday life scenarios where we
try to reduce friction? Where we need friction?
Coefficient of Friction
μ has NO units because it is a ratio
The above equation can be rearranged to give
the equation for Ff:
In general,
Static friction > kinetic friction
Therefore, we have two different coefficients:
Coefficients must be determined experimentally.
Approximate Coefficients of Friction
Materials in Contact
μK
μS
Oak on oak, dry
0.30
0.40
Waxed hickory on dry snow
0.18
0.22
Steel on steel, dry
0.41
0.50
Steel on steel, greasy
0.12
Steel on ice
0.010
Rubber on asphalt, dry
1.07
Rubber on asphalt, wet
0.95
Rubber on concrete, dry
1.02
Rubber on concrete, wet
0.97
Rubber on ice
0.005
Leather on oak, dry
0.50
Some different examples on page 170 in text
Example 1
Determine the appropriate coefficient of friction in
each case.
a) It takes 59 N of horizontal force to get a 22 kg leather
suitcase just starting to move across the floor. (static)
b) A horizontal force of 54 N keeps the suitcase
in (a)moving at a constant velocity. (kinetic)
Example 2
A 73 kg hockey player glides across the ice on skates
with steel blades (constant velocity). What is the force
of friction on the skater?
Homework
Pg 171 #’s 1,3
Pg 172 #’s 1, 3-8