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Friction Forces
AP Physics C
Mrs. Coyle
Outline
• Static Friction
• Kinetic Friction
• Coefficient of Friction (Static and Kinetic)
Friction: the resistance to an object’s motion
when it travels against a surface or in a viscous
medium. It has a direction parallel and opposite
the direction of motion.
• Static Friction, fs : the frictional force that exists
when a force is applied to attempt to move an object
across a surface but the object is not yet set in
motion; fs counteracts the applied force.
• Kinetic Friction, fk : the frictional force that exists
when an object has been set in motion across a
surface.
Active Figure- Static and Kinetic Friction
• http://www.csupomona.edu/~skboddeker/132/note
s/Active%20Figures%20Media/AF_0516.swf
Coefficient of Friction, µ
• fs ≤ µs N (the equality holds in the case of
impending motion).
• fk= µk N
• Values of µ depend on the surfaces and are
usually between 0.03 and 1.0
• µ has no units
Active Figure-Incline Plane With and Without Friction
• http://www.csupomona.edu/~skboddeker/132/note
s/Active%20Figures%20Media/AF_0519.swf
Example 1: Find the coefficient of friction as a
function of the angle ɵ, for the inclined plane shown
a) for the case where the object is moving at constant
velocity
b)where the object is moving at a constant
acceleration.
ɵ
Example 2: A 1.00kg book is held against a wall by
pressing it against the wall with a force of 50.00N.
What must be the minimum coefficient of friction
between the book and the wall, such that the book
does not slide down the wall?
F