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Chapter 6

PHY 101 Chapter Notes - Chapter 6: Net Force, Acceleration, Turning Radius

Course Code
PHY 101
Dr.sharon Zane

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Chapter 6
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
friction is helpful in some situations (starting to walk, turning the corner while
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
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
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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