Friction
MR. VILLA
PHYSICS
Friction
 What is Friction?????
 When two surfaces are in contact, friction forces
oppose relative motion or impending motion.
 Friction forces are parallel to the surfaces in contact
and oppose motion or impending motion.
P
Coefficient of Friction
 Every surface has different degrees of friction
 These different surfaces have a measurement of
“how much” friction they have, we use the symbol
m (mu) to determine a specific friction from a
specific surface……….
Friction and the Normal Force
 The force required to overcome static or kinetic
friction is proportional to the normal force, Fn.
Fn 12 N
Fn 8 N
Fn 4 N
2N
Fs = msFn
4N
F k = mk F n
6N
There are 2 types of Friction
 Static Friction
 Kinetic Friction
 No Relative motion
 Relative Motion
 Static Friction will resist
 Once an object is in
motion and counteract
any applied force up to a
certain maximum where
friction is overcome and
motion begins.
F s = ms F n
motion, kinetic friction
will resist motion. This
resistance is independent
of velocity.
Fk = mk F n
Friction Continuing….
Friction forces are independent of area
 If the total mass pulled is constant, the same force (4
N) is required to overcome friction even with twice
the area of contact.
 For this to be true, it is essential that ALL other
variables be rigidly controlled.
4N
4N
Friction Forces
 Heat can sometimes cause surfaces to become
deformed or sticky. In such cases, temperature can
be a factor.
Friction forces are independent of
temperature, provided no chemical or
structural variations occur.
4N
4N
Friction forces are independent of speed.
 The force of kinetic friction is the same at 5 m/s as it
is for 20 m/s. Again, we must assume that there are
no chemical or mechanical changes due to speed.
20 m/s
5 m/s
2 N
2 N
Friction and Acceleration
When Fa is greater than static friction, the
resultant force produces acceleration. However,
as soon as the object begins moving, friction
decreases to the kinetic frictional force
FS
a
Fa
FS  ms FN
Fk  mk FN
Note that the kinetic friction force remains
constant even as the velocity increases.
Example 1
 A hockey puck has a coefficient of friction of μk = .10.
If the puck feels a normal force (Fn) of 5 N, what is
the frictional force that acts on the puck?
F k = mk F n
Fn
Fk
Fa
 Fk =(.10)(5)
 Fk
=.5
Fg