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● 6-1 Frictional Forces

● friction: the force required to overcome the resistance of microscopic hills and

valleys and bumping together

● even smooth surfaces are jagged at the atomic level which causes resistance to

an objects motion; the resistance is the force of friction

● friction is thought of as something that should be reduced or eliminated if

possible

● friction is helpful in some situations (starting to walk, turning the corner while

driving)

● Kinetic Friction

○Kinetic friction- the friction encountered when surfaces slide against one

another with a finite relative speed

○fk acts to oppose the sliding motion at the point of contact between the

surfaces

○When the normal force (N) is doubled fk is also doubled; fk is proportional

to the magnitude of N.

○fk= kN

■ k is referred to as the coefficient of kinetic friction, it is the

constant of proportionality

○N in special cases equals the weight of the object overcoming friction but if

someone pushes down on the object N becomes greater than the

objects weight which would increase friction

○N is less than the weight of the object if it is on an incline

○The force of friction opposes motion and is thus not a vector equation b/c N

is perpendicular to the direction of motion

○When an object is pulled at speed v and then later 2v, the force of kinetic

friction is about the same in each case; it does not double fk is

independent of the relative speed of surfaces

○If the area of contact of an object is reduced, the force of friction remains

the same regardless of the area of contact; fk is independent of the

area of contact b/t the surfaces

● Static Friction

○Static friction- the friction that keeps two surfaces from moving relative to

one another due to the microscopic irregularities of surfaces that are in

contact

○Typically stronger than kinetic friction b/c the hills and valleys of each

surface can nestle into one another

○When an object is motionless, the fs is zero

○When a force f1 attempts to pull an object and it does not move, f1=fs; if f1

increases and the object still does not move, fs has also increased to the

same value

○When the object starts moving, f s is overcome and fk takes over; the upper

limit that fs reaches is called fs,max > fs > 0

○fs,max= sN

■ s is the coefficient of static friction, the constant of

proportionality; it is usually greater thank k showing that fs > fk

○fs is independent of the area of contact between the two surfaces

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