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Friction & Applying Newton’s 2nd
Law
System
Chapter 6.2
What is Friction?
• Friction is a
that is parallel to the
surfaces of two objects that are in contact
with one another that
the relative
of the two objects.
Ff
What causes friction?
• Friction is caused by the temporary
created
between two objects in contact with
one another.
Friction
• How does friction affect the motion of
objects?
– It can
an object down
like the friction between
the tires and the road.
– It is responsible for
the speed of an
object like a car.
– It is also responsible
for objects being able to
.
Objects)
Friction (
• Static Friction:
– The
from moving.
– Since v = ___, a = ___.
that keeps an object
Fground-on-crate
Ffriction
FApplied
Fnet =
Since the crate is not accelerating, Fnet = __
=
System
Fgravity
Note: As long as the crate does not move, _____ = _____
Friction (Objects in
)
• Kinetic Friction:
– The
that opposes
the relative motion of two contacting surfaces that
are
past one another.
– Fnet may or may
be ___.
Fground-on-crate
Fnet
Fapplied
Ffriction
System
Fgravity
Fnet =
Note: If the crate moves at a
, then _____ = _____ and
Fnet = ___.
Determining the Frictional Force
(The
of Friction, __)
The force of friction (Ff) is proportional to the normal force (FN)
and a proportionality constant (___ - pronounced _____) called
the
of friction.
FN
• For static friction:
– 0 < Ff, static < sFN
f
• For kinetic friction:
– Ff, kinetic =  kFN
F
•
F 
f
• As per the formula, the greater ___, the greater the
.
•
Note: FN = the
to the
• __ is dimensionless (
•
_____ > _____
(
on the object.
)
)
The
Force
• The
force is a force that most
often opposes the Earth’s gravitational
attraction and is
to the
that an object rests or is moving on.
– For a
surface, ____ = ____ = ____.
– For a surface that is
as
seen in the figure below, _____ = ________.
FN

The Normal Force
FN
Fg
FN

cos = adj/hyp
F
g
____ = ____ = ____
____ = ________ = ________
Example 2: Determining Friction
(Balanced Forces)
•
Assume that the man in the figure is
pushing a 25 kg wooden crate across a
wooden floor at a constant speed of 1 m/s.
–
How much force is exerted on the crate?
FN
Fapplied
Ff
System
Fg
Diagram the Problem
+y
System
y-direction: ____ = ____
x-direction: Fnet =
+x
Since the crate is moving with
a = ___, Fnet = ___, and ______ = ______
speed,
State the Known and Unknowns
• What is known?
o Mass (m) =
o Speed =
o Acceleration (a) =
o k =
• What is not known?
o Fapplied = ?
Perform Calculations
• y-direction:
o ___ = ___ = ___ (Note:
surface)
• x-direction: Since a = ___, Fnet = ___
o Fnet =
–
Example 3: Determining Friction
(
Forces)
•
Assume that the man in the figure is pushing a
25 kg wooden crate across a wooden floor at a
speed of 1 m/s with a force of 73.5 N.
–
If he doubled the force on the crate, what would
the acceleration be?
FN
Fapplied
Ff
System
Fg
Diagram the Problem
+y
System
y-direction: ____ = ____
x-direction: Since
, Fnet =
+x
State the Known and Unknowns
• What is known?
o Force =
o Mass (m) =
o Speed =
o k =
• What is not known?
oa?
Perform Calculations
• y-direction:
• x-direction:
o Fnet =
o ma =
oa=
Key Ideas
• Friction is an
that
exists between
bodies.
• Friction is
to the
and the
of
; static or kinetic.
• The force required to overcome static
friction is
than that
required to overcome kinetic friction.