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Friction
The force of_________________,
F f:
1. Ff = A ____________
force that ____________
contact
opposes
motion. It is caused by the __________________
breaking and
______________________________________
re-forming of electromagnetic bonds between
its environment
an object and _________________________.
2. Friction usually results in __________
heating when
molecules in
motion energy is transferred to ______________
the object and its environment. This results in a
temperature _____________
phase change
increase or a _________________,
such as _____________.
melting
Ex: car
skidding
on road
kinetic energy
________
tire
heats up ______
road
and _________
molecules
Three basic types of friction:
A._____________
friction– due to collisions with
fluid
fluid molecules; increases with speed
water or air resistance
Ex: _________________________________
rolling
B._____________
friction –reduces amount of
friction because there is less __________________
surface contact
Ex: ___________________
ball bearings
flat
C. ______________
friction – when two _______
sliding
surfaces slide past each other; what we
_____________
are going to learn most about.
most
least
sliding  ___________
rolling
fluid
force  __________
 __________
 force
Sliding Friction:
All About ___________
A 10-N object at rest:
Forces acting on it:
FN = 10 N
10 N
0
Fnet =____
w = 10 N
Now pull it with a 2 N
force but it doesn’t move:
10 N
pull
Fp = 2N
0
Fnet =____
FN = 10 N
Ff = 2N
Fp = 2 N
w = 10 N
Pull it with a 4 N force
and it still doesn’t move:
FN = 10 N
Ff = 4 N
pull = 4 N
10 N
0
Fnet =____
w = 10 N
When you increase the force
accelerates
to 6.0 N, it _______________:
10 N
pull = 6 N
Fnet ___
≠ 0
Fp = 4 N
Ff < 6N
FN = 10 N
Fp = 6 N
w = 10 N
maximum
__________
Ff,max
static
_______ friction = _____
5.0 N
Once it starts moving, you only need a _______
constant velocity
pull to keep it moving at _____________________:
10 N
FN = 10 N
pull = 5 N
Ff = 5 N
0
Fnet = ____
Fp = 5 N
w = 10 N
kinetic
__________
Ff , k
(moving) friction = _____
__________
kinetic
__________
friction Ff,k
<
maximum
___________
static Ff,max
Note:
Static
1._____________
friction, Ff
at rest
friction when body is_______________
any amount up to _______________
maximum
can be _____
static friction
Ff,max
2. Maximum static friction, __________
breaks free
friction just before the object ________________
electromagnetic
During static friction, the _________________
bonds between the object and the
molecular _________
stretching
surface are ________________
microscopically, at
more and more As the
first a little, then __________________.
breaking
bonds reach the ________________
point,
maximum static
_______________________
friction occurs.
3. _________
Kinetic friction, Ff,k is the friction that occurs
constant velocity
when an object moves at ____________________
.
 always ___
< the__________
max. static friction: Ff,max
bonds are
 As it slides, the surface _________
breaking and________________
re-forming
constantly ____________
Ex: Same wooden box
a/ On different sides:
v
v
same F
__________
f,k
b/ At different speeds:
v
v
same
__________
Ff,k
Ff,k is roughly independent of
surface area , and
a/ ________________
speed
b/ _______________
.
A graphical view of static vs. kinetic friction:
Ff
bonds
break
______
Ff,max
Static
Ff can
___
any
be ____
value
in here
break
bonds ____________
re-form
and ______________
Ff,k
Static Friction
_________
Ff,k
Ff,max > _______
Static _______
Kinetic
____________
Friction
In either static or kinetic friction, sliding friction
depends on two factors:
nature of the surfaces
1. The____________________________
– waxed skis
on snow, rubber on dry concrete, etc
LESS
Usually, smoother surfaces  __________
friction
presses the surfaces together
2. The force that________________________________
normal
 This force is the ____________
force: ______
FN
But press down
on the block:
Block on
a table:
FN
F
w
w
FN = ______
FN
w+F
FN =_________
w
F
These 2 factors are summed up in the equation:
Ff = mFN
coefficient
 m is called the _________________
of friction,
mu ” It:
and is the Greek letter “______.
roughness on Ff
- describes effect of surface ______________
bigger
- is usually _____________
for rougher surfaces
Ff/FN  units _________
cancel
- has ____
no units: m = _______
- has different values for:
ms
1/ maximum static friction: _____
mk
2/ kinetic friction : _____,
ms > ____
mk .
and with ____
From the PhysRT, page 1
___ :
Notice:
1.This should say
___________
Maximum Static
2.
mk
mk ___
< ms
ms
For maximum static Ff,max, use:
Ff = ____
ms FN
mk N
For kinetic Ff,k, use:
Ff = ____F
Ex: A 12,000-N crate is on waxed skis on snow.
Find the force needed to…1/ start it moving, and
2/ keep it moving at constant velocity
FN = 12,000 N
w = 12,000 N
1/ max. static: Ff = mFN = msFN
= (0.14) (12,000 N)
= 1700 N
Ff = mFN = mkFN
2/ kinetic:
=
=
(0.05) (12,000 N)
600 N
How would your answers change if another
2000 N were added to the sled?
Open a textbook to the
page 145.
Do: #1,2 and 3
no pull
no friction
no stretching
of bonds
slight pull
some Ff
some
stretching
just before it
breaks free: Ff,max
maximum
stretching