FRICTION
Is F = μN
What is Friction?
Resistive force caused by two surfaces
rubbing against each other
 Resists an object's motion - acts opposite
direction of motion or "intended motion"
 Depends on materials, texture of surface
(smooth or rough) caused by microscopic
or sometimes macroscopic irregularities in
surface (“bumpiness”)
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Types of Friction
· Kinetic: two surfaces sliding against each
other, object sliding against a surface (ex:
sledding, skiing, snowangels, car brakes,
slide at playground, rubbing hands together)
· Static: friction between a surface and a nonmoving object
Sliding Friction
Static Friction
 When an object is at rest and another object is
attempting to push/pull it across a surface
 Gradually increases to a maximum value, peak static
friction (Fs,max)
Kinetic Friction (Fk)
 When an object is already in motion (sliding across a
surface)
 Generally a constant value, Fk
Fs,max > Fk
What affects friction?
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Coefficient of Friction (Greek letter μ “mu”) is a
measurement of how tacky or smooth 2 surfaces are
units: unitless, just a number
the greater the magnitude, the tackier or rougher
the surfaces are
F = μN
Table of μ
Try It Yourself!
2. A 25 kg chair initially at rest on a horizontal floor requires a 165 N horizontal force
exerted by a person to set it in motion.
a. Find the coefficient of static friction between the chair and the floor.
Solution
3. Once the chair is in motion, a 127 N horizontal force exerted by the person keeps it
moving at a constant velocity.
a. Find the coefficient of kinetic friction between the chair and the floor.
FD Example
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A 10 kg crate is being pulled by a person across the factory floor
with an acceleration of 0.5 m/s². If the coefficient of kinetic friction
between the crate and the floor is 0.35, how much does force does
the person have to exert on the crate?
ΣFx = FPonC + FfGonC
ΣFy = FGonC + FEonC
FGround on
ax = [FPonC + μ*FGonC]/m
ax = [FPonC + μmg]/m
FPonC = max + μmg
FPonC = (10kg*0.5 m/s²)
+ (-0.35*10kg*10N/kg)
FPonC = 40 N
Crate
F(friction)Ground on
Crate
FPerson on
Crate
FEarth on Crate