Download The Effects of Friction

THE EFFECTS OF FRICTION
FRICTION
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Friction resists motion and
acts in a direction opposite to
the direction of motion.
It occurs because of the
electrical forces between the
surfaces where two objects are
in contact.
STATIC FRICTION AND KINETIC FRICTION
 Static
friction is the force that tends to
prevent a stationary object from starting to
move.
 The maximum static friction is called the
starting friction.
 Kinetic friction is the force that acts against
an object’s motion in a direction opposite to the
direction of motion.
 Sliding
friction affects a toboggan; rolling
friction affects a bicycle; and fluid friction
affects a boat moving through water and an
airplane flying through air.
 The Effect of friction is heat and loss of
efficiency
 Unwanted friction can be reduced by changing
sliding to rolling, by polishing by using
bearings, and by using lubrication.
COEFFICIENT OF FRICTION
 By
experiment, the coefficient of friction can
tell a physicist how much friction will be
present between an object and a surface.
 Coefficients are ratios of the frictional force to
the normal force.
COEFFICIENT OF STATIC FRICTION
 Every
two contacting objects have unique
properties and therefore a different coefficient
of static friction:
COEFFICIENT OF KINETIC FRICTION
 Every
two contacting objects have unique
properties and therefore a different
coefficient of kinetic friction:
MEASURING COEFFICIENT OF FRICTION
=
=
tan θ
COMMON COEFFICIENTS
Materials in
Contact
Kinetic
Coefficient
Static Coefficient
Wood on wood
0.2
0.25-5
Steel on wood
0.25
0.45
Steel on ice
0.010
0.10
Rubber on dry
asphalt
1.0
1.1
Rubber on wet
asphalt
0.95
1.0
•More on page 101 in your textbook.
EXAMPLE # 1
It takes 50N to pull a 6.0 kg object along a
desk at constant speed. What is the coefficient
of friction?
EXAMPLE #2
The coefficient of sliding friction between two
materials is 0.35. A 5.0 kg object made of one
material is being pulled along a table made of
another material. What is the force of friction?
EXAMPLE # 3
A packing crate is placed on a plane
inclined at an angle of 35° from
horizontal. If the coefficient of static
friction between the crate and the plane is
0.65, will the crate slide down the plane?
EXAMPLE # 4
A piece of wood is sliding on a steel table
saw (mk=0.25) is pushed with a force of
50N. If the wood is moving at constant
velocity, what is the mass of the wood?
EXAMPLE # 5
A 800-N student is Rollerblading at a
velocity of 6m/s [W]when the student trips
and slides horizontally along the trail,
coming up to stop in 1.5 seconds.
a. Determine the Student’s average
acceleration.
b. What is the kinetic friction as the
student slides?
c. Determine the coefficient of kinetic
friction between the student and the trail.