Chapter 4
Friction
Forces of Friction

When an object is in motion on a surface
or through a viscous medium, there will
be a resistance to the motion
• This is due to the interactions between the
object and its environment

This is resistance is called friction
• Surfaces
• Air friction: bullet, jet
• Fluids: swim
Static Friction, ƒs




Static friction acts
to keep the object
from moving
If F increases, so
does ƒs
If F decreases, so
does ƒs
ƒs  µs FN
f
max
s
  s FN
Kinetic Friction, ƒk


The force of kinetic
friction acts when
the object is in
motion
ƒk = µk FN
• Variations of the
coefficient with
speed will be
ignored
Friction Demo



Friction force always opposes motion
Book will not slide unless pushing
max
force is larger than certain value f s
Once book starts to slide, there is
less friction
max
fk  fs
More About Friction





Friction is proportional to the normal force
The force of static friction is generally
greater than the force of kinetic friction
The coefficient of friction (µ) depends on
the surfaces in contact
The direction of the frictional force is
opposite the direction of motion, tangent
to sliding surface
The coefficients of friction are nearly
independent of the area of contact
Example
How large a force needed to start a
4kg wood block moving? Keep it
moving?
Table 4.1 in the book gives
 k  0.3
 s  0.5
for wood on wood
Example
A car hits another car in an accident.
Damage to the cars show that the
speed of impact is 55 mph. There is
also 25 feet of skid marks. If the
speed limit was 65 mph, was the first
car speeding? (k=0.8)
Example
Woman pulls sled at an angle 42° with
constant velocity along a flat road.
(k=0.1)
What is the tension in the pulling
rope?
Inclined Planes




Fig. 4.4 is very important
Modify Fig. 4.4 for friction
Choose the coordinate system with x
along the incline and y perpendicular
to the incline
Replace the force of gravity with its
components
Example
Block being pulled up a hill. What
tension must rope have before block
moves? (m=10kg, =25°, s=0.6)
Example (prob. 29)
m1=3kg
m2=2kg
k =0.3
Find acc. and
tension.
Terminal Velocity



Object in free fall will actually come
to constant velocity due to friction
forces from air.
Air friction forces increase with
velocity
Depends on size & shape of object
Terminal Velocity