Download Force of Friction

Force of Friction
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Friction acts to oppose motion between
two surfaces in contact
Ff
Dependant on
– Surfaces
– Normal Force
FF
Causes of friction:

When two surfaces touch, they
temporarily bond. In order to move
the object one must break this bond.
When objects are moving past each
other, there is still an electrostatic
attraction at the atomic level and this
is the weaker kinetic friction.
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Static friction:
– opposing force exerted by a surface when
there is no relative motion
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Kinetic friction:
– opposing force exerted by a surface when
there is relative motion
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Ff kinetic = μk FN
μ is a constant dependant on the surfaces
in contact.
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Some common coefficients are in table 4-2, pg 97
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0 ≤ Ff
static
≤ μs FN
Ff kinetic = μk FN
or
Example

You push a 25 kg wooden box on a wooden floor with a
constant velocity of 1.00m/s. How much force do you
exert on the box? μk = 0.2
m=25kg
Ff = μk FN
v=1.00m/s
FN = Fg =mg
μk = 0.2
So substituting:
Fa= ?
Constant Velocity ΣF = 0
ΣFx = 0 = Fa – Ff
Fa = Ff
Ff = μk mg
Ff = 0.2 (25kg) (9.80m/s2)
Ff = 5N
Fa = 49N