Chapter4
RESISTANCE
Friction Forces
Friction- a force that opposes
motion of two solids or a solid
and a fluid
Static Friction (Fstatic) - a
force required to start
relative motion between two
objects
Kinetic Friction (Fkinetic) the force required to
maintain a constant speed
between two surfaces in
contact
WHAT CAN BE DONE TO
DECREASE THE AMOUNT OF
FRICTION EXPERIENCED BY
TWO OBJECTS?
Static friction is usually greater than the
kinetic friction.
This is due in part as a result of how
hard the surfaces are pressed together.
This is called the normal force (FN)
Coefficient of Friction (μ)
The ratio of maximum force
(static or kinetic) of friction to
the normal force between the
two surfaces in contact.
Surface
μS
μK
Wood on wood
0.5
0.2
Wood on concrete
0.6
0.4
Rubber on dry concrete
0.8
0.7
Rubber on wet concrete
0.65
0.57
Steel on steel (dry)
0.78
0.58
Steel on steel (oiled)
0.11
0.05
Steel on Teflon
0.04
0.04
Lab Preview
A spring scale will be
used to determine the
pulling force required to
get the sled to move?
Determining Kinetic Coefficients
of Friction
Lab Preview
Surface
Mass of Sled
(kg)
Mass Added
(kg)
Ivory on Dry
Metal
0.08
.250
Total Mass
(kg)
Weight of Sled
and Mass
(Newtons)
Lab Preview
Surface
Kinetic Friction
Force
Normal Force
Ivory on Dry
Material
0.42
2.53
Coefficient of
Kinetic Friction
μK = (Kinetic Friction Force) / Normal Force