Net Forces, Friction,
Coefficients of
Friction
Sliding Book Example
• Why do things not continue to move at
constant velocity?
Friction
• If the sliding book slows down, what’s
the force responsible?
Friction
– How could I keep it moving at a constant
velocity?
Remove Friction or continue to add
force equal to friction
Create a net force of zero
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The three types of friction
1) Surface or Sliding Friction – Created when an object
slides or moves over a second object while their two
surfaces are touching.
2) Fluid Friction- Created when an object slides or moves
over a second object that is a fluid (Gas or liquid). This
reduces the amount of Peaks & Valleys that will interact
while their two surfaces are touching.
3) Rolling Friction - Created when an object rolls over a
second object. The circular motion constantly refreshes
the two surfaces that are coming into contact.
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Friction opposes Motion
Surface Friction
Acceleration
Force on box
by person
Force on floor by box
Force on person
by box
Force on box
by floor
What kind of motion is created by an Unbalanced Force?
It’s the sum of all the forces that determines the type of motion.
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Friction due to the Surface
How does the corrugated surface change Friction?
Corrugations in the surfaces grind when things slide.
Lubricants fill in the gaps and let things slide more easily.5
Why Doesn’t Gravity Make the Box Fall?
Force of Floor acting on Box
Force from floor
on box
and gravity = Net
Force of zero.
If the floor
vanished, the
box would fall,
Net force  zero
Force of Earth acting on Box (weight)
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What’s Forces are not shown?
Force on box
by person
Force on floor by box
Force on person
by box
Force on box
by floor
The Normal Force and the Force of Gravity
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• When we drew the box and floor,
with the “normal” force and the
force of gravity, these weren’t
strictly force pairs
– forces on the box that result in a Net
zero acceleration of the box
• The real pairs have to involve the
earth:
box
floor
• Force Pairs:
• earth-box (grav)
• box-floor (contact)
• earth-satellite (grav)
satellite
earth
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Questions
• Does friction always exert a force?
NO
Friction vector is
• What does this say about the
opposite in
direction of the frictional force,
direction of
relative to the velocity vector?
motion
• What do you think would happen if
we loaded lead bricks into the box?
Yes
Would it become harder to slide?
• What are some ways to reduce
frictional forces?
Lubrication, change surface,
reduce normal force
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Static and Kinetic
Friction
Static frictional force: when two surfaces
are touching and nothing is sliding, No
motion
(ƒs)
Kinetic frictional force: when two surfaces
are touching and at least one of the
surfaces are sliding or moving over the
other.
(ƒk)
Static frictional forces are always greater than Kinetic
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Friction is
ƒ=µ N
Friction = (Coefficient of Friction)
(Normal Force)
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“Normal” Forces and
“Normal” means
Frictional Forces perpendicular
Reaction Force
From Ramp
Normal
Force
Friction
Force
Decompose Vector
Weight of block
Weight of block
Friction Force = Normal Force  (coefficient of friction)
Ffriction = Fnormal
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Coefficient of Friction
Material on
Material
steel / steel
add grease to steel
metal / ice
brake lining / iron
tire / dry pavement
tire / wet pavement
s = static friction
0.6
0.1
0.022
0.4
0.9
0.8
k = kinetic friction
0.4
0.05
0.02
0.3
0.8
0.7
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Stay on the road!
• What does it take to stay on the road
around a curve?
– using s = 0.8 as average for tires on road,
Ffriction = 0.8mg
• (Normal force is just mg on level surface)
Fcurve = macurve = mv2/r
• where r is radius of curve, say 50 m (e.g., cloverleaf
exit ramp)
• Got enough friction if Fcurve < Ffriction
– happens if v2 < 0.8gr, or v < 20 m/s = 44 m.p.h.
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