2. The force of_________________, Ff:
Ff = A ____________ force that ____________
motion. It is caused by the __________________
______________________________________ between
an object and _________________________.
Friction usually results in __________ when
motion energy is transferred to ______________ in
the object and its environment. This results in a
temperature _____________ or a _________________,
such as _____________.
Friction
opposes
contact
breaking and
re-forming of electromagnetic bonds
its environment
heating
molecules
increase
phase change
melting
kinetic
________ energy
heats up ______
and _________
molecules
Ex: car
skidding
on road
tire
road

3. Three basic types of friction:
_____________ friction– due to collisions with fluid molecules; increases with speed
Ex: _________________________________
_____________ friction –reduces amount of friction because there is less __________________
Ex: ___________________
C. ______________ friction – when two _______ _____________ slide past each other; what we are going to learn most about.
fluid
water or air resistance
rolling
surface contact
ball bearings
sliding
flat
surfaces
most least
force  __________  ___________  __________  force
sliding
rolling
fluid

4. All About ___________ Friction:
Sliding
All About ___________ Friction:
A 10-N object at rest:
Forces acting on it:
10 N
FN = 10 N
Fnet =____
w = 10 N
Now pull it with a 2 N
force but it doesn’t move:
FN = 10 N
pull
Fp = 2N
10 N
Ff = 2N
Fp = 2 N
w = 10 N
Fnet =____

5. ≠ Pull it with a 4 N force and it still doesn’t move: 10 N Fnet =____
FN = 10 N
Ff = 4 N
10 N
pull = 4 N
Fp = 4 N
Fnet =____
w = 10 N
When you increase the force
to 6.0 N, it _______________:
FN = 10 N
accelerates
Ff < 6N
Fp = 6 N
10 N
pull = 6 N
w = 10 N
Fnet ___ 0 ≠
maximum
static
__________
_______ friction = _____
Ff,max

6. < Once it starts moving, you only need a _______ 5.0 N
pull to keep it moving at _____________________:
5.0 N
constant velocity
FN = 10 N
10 N
pull = 5 N
Ff = 5 N
Fp = 5 N
Fnet = ____
w = 10 N
kinetic
(moving)
__________
__________ friction = _____
Ff,k
kinetic
___________
static Ff,max
maximum
__________
friction Ff,k
<

7. Note:
_____________ friction, Ff
friction when body is_______________
can be _____ amount up to _______________
static friction
Maximum static friction, __________
friction just before the object ________________
Static
at rest
any
maximum
Ff,max
breaks free
During static friction, the _________________ molecular _________ between the object and the surface are ________________ microscopically, at first a little, then __________________. As the bonds reach the ________________ point, _______________________ friction occurs.
electromagnetic
bonds
stretching
more and more
breaking
maximum static

8. 3. _________ friction, Ff,k is the friction that occurs
when an object moves at ____________________ .
 always ___ the__________ static friction: Ff,max
 As it slides, the surface _________ are
constantly ____________ and________________
Kinetic
constant velocity
<
max.
bonds
breaking
re-forming
Ex: Same wooden box
b/ At different speeds:
a/ On different sides: v v v v
__________ Ff,k
same
__________ Ff,k
same
Ff,k is roughly independent of
a/ ________________ , and
b/ _______________ .
surface area
speed

9. A graphical view of static vs. kinetic friction:Ff
bonds
______
break
break
bonds ____________
and ______________
Ff,max
re-form
Static
___ can
be ____
value
in here Ff
stretching
any
Ff,k
bonds _____________
Static
_________ Friction
____________ Friction
Kinetic
Ff,max
Ff,k
Static _______ > _______

10. In either static or kinetic friction, sliding friction
depends on two factors:
The____________________________ – waxed skis
on snow, rubber on dry concrete, etc
Usually, smoother surfaces  __________ friction
The force that________________________________
 This force is the ____________ force: ______
nature of the surfaces
LESS
presses the surfaces together
normal FN
Block on
a table:
But press down
on the block: FN FN F w F w
w + F
FN = ______ w
FN =_________

11. These 2 factors are summed up in the equation:
Ff =
mFN
 m is called the _________________ of friction,
and is the Greek letter “______. ” It:
- describes effect of surface ______________ on Ff
- is usually _____________ for rougher surfaces
- has ____ units: m = _______  units _________
- has different values for:
1/ maximum static friction: _____
2/ kinetic friction : _____,
and with ____ > ____ .
coefficient mu
roughness
bigger
Ff/FN no
cancel ms mk ms mk

12. From the PhysRT, page ___ :1
Notice:
This should say
___________ Static
2. mk ___ ms
Maximum
< mk ms ms
For maximum static Ff,max, use: Ff = ____ FN
For kinetic Ff,k, use: Ff = ____FN mk

13. 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
2/ kinetic:
Ff = mFN
mkFN = =
(0.05)
(12,000 N) =
600 N
How would your answers change if another
2000 N were added to the sled?