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Frictional Forces
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Friction
• Friction is a force that opposes motion
• Friction will cause a moving object to
slow down and finally stop.
• If ANY two objects are touching, there
is friction!!!!
Friction
Types of Friction
• Sliding Friction – occurs when object
slide over each other.
• Example: Sliding your chair back and
forth
• Rolling Friction – Produced by objects
such as wheels or ball bearings.
• Example: A car going down the road or a
cart being pushed down the hallway
• Kinetic friction: the friction
experienced by surfaces sliding against
one another
• Static friction: force depends on the
normal force:
Kinetic Friction
Moving Friction
The constant μk is called the coefficient of
kinetic friction.
Kinetic Friction
Kinetic Friction
The kinetic frictional force is also
independent of the relative speed of the
surfaces, and of their area of contact.
• Low Load
• High Load
• Elastic
deformation
Static Friction
• The static frictional force keeps an object
from starting to move when a force is applied.
• The constant μs is called the coefficient of
static friction.
• The static frictional force has a maximum
value, but may take on any value from zero to
the maximum
Static Friction
Static Frictional Force
Static Friction
• The static frictional force is also
independent of the area of contact and
the relative speed of the surfaces
2
Question?
• A desk has a mass of 27.5 kilograms. If the
coefficient of static friction between the
desk and the floor is 0.44, what force must
be used to move the desk from rest?
• f s = μs N
•
•
= .44 * (27.5*9.8)
= 118.6N
N = m *g
Question
• Once the desk above is in motion, what force
must be used to keep it moving at a constant
velocity if the coefficient of kinetic friction is
0.293
• FN = (m*g)
• FN = (27.5*9.8)
• fs = μk FN =.293 * (9.8*27.3)
• = 78N
Extra Question
• 3) An object takes 12.3 meters to stop
because of friction. Assume no skidding. If
the coefficient of static friction is 0.44,
what is the initial velocity of the object?
• A ninja of mass 70 kg is at rest on a ramp
with slope of 75 degrees. What is the
frictional force applied to the ninja?
Are we dealing with static friction
or kinetic friction?
Remember: fs = μs N
70 degrees
Free Body Diagram
70
Lab
• Purpose: find the force of static friction acting
upon your object and coefficient of static friction
• Materials: Protractor, ramp, object
• Be sure to include a free body diagram, 5 trails
in which you average your measured max
angles, data analysis, and RERUNN conclusion