Friction.notebook
February 17, 2015
Friction
The force of friction always opposes the motion of an object. It "always" acts opposite to the direction of the motion or attempted motion.
Ff α Fg
The force of friction is proportional to the weight of the object. i.e. it is harder to slide a heavy piece of furniture across a floor than a lighter one.
Ff α FN
The force of friction is proportional to the normal force on the object, since FN and Fg are related to one another.
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Friction.notebook
February 17, 2015
Mathematically,
Ff = μFN
Where Ff is the force of friction in Newtons, FN is the normal force in Newtons and μ is the coefficient of friction (no units).
μ depends on the two surfaces in contact. It does not depend on the speed of the objects or the area in contact.
Depending if the objects are moving or stationary we must consider two coefficients of friction.
μs static friction coefficient ~ friction keeping an object stationary
μk kinetic friction coefficient ~ friction between moving objects
Generally Fs ≫Fk (car in mud example)
When there is no external force, there is no static friction.
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Friction.notebook
February 17, 2015
Example: A small cardboard box of mass 12 kg is pushed across a level floor. It slides a distance of 7 m coming to rest in 3.1 s. Find the coefficient of friction between the box and the floor.
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Friction.notebook
February 17, 2015
Example: Miss Takken pushes her 50 kg washing machine across the floor at a constant speed. If the coefficient of friction is .25,
a. What is the frictional force on the washer?
b. What is the applied force that Miss Takken puts on the washer?
pg 48 # 1­3
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