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The force of_________________, Ff:
1. Ff = A ____________ force that ____________
motion. It is caused by the ________________________________________________________ between an object and _________________________. 2. 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
heating
increase
molecules
phase change
Ex: carskiddingon road
________ energyheats up ______and _________molecules
kinetictire
road
contactbreaking and
re-forming of electromagnetic bondsits environment
melting
Three basic types of friction:A._____________ friction– due to collisions with fluid molecules; increases with speed Ex: _________________________________B._____________ 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
rolling
most leastforce __________ ___________ __________ force
ball bearings flat
surfacessliding
water or air resistance
sliding rolling fluid
surface contact
A 10-N object at rest:
10 N
Forces acting on it:
w = 10 N
FN = 10 N
Now pull it with a 2 N force but it doesn’t move:
10 N
w = 10 N
FN = 10 N
pull Fp = 2N Fp = 2 NFf = 2N
Fnet =____
Fnet =____
All About ___________ Friction:Sliding
0
0
_________________ friction = _____
Pull it with a 4 N forceand it still doesn’t move:
10 N
w = 10 N
FN = 10 N
pull = 4 N Fp = 4 NFf = 4 N
Fnet =____
When you increase the forceto 6.0 N, it _______________:
10 N
w = 10 N
FN = 10 N
pull = 6 N
Fnet ___ 0
Fp = 6 N
maximumstatic
0
≠
accelerates
Ff < 6N
Ff,max
Once it starts moving, you only need a _______ pull to keep it moving at _____________________:
10 N
w = 10 N
FN = 10 Npull = 5 N
Ff = 5 N
Fnet = ____
Fp = 5 N
Ff,k
___________
static Ff,max
__________
friction Ff,k
<
5.0 Nconstant velocity
0
____________________ friction = _____kinetic(moving)
kinetic maximum
Note:
1._____________ friction, Ff
friction when body is_______________can be _____ amount up to _______________ static friction
2. Maximum static friction, __________ friction just before the object ________________
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.
at restStatic
any maximum
Ff,max
breaks free
electromagnetic
stretchingbonds
more and morebreaking
maximum static
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________________
Kineticconstant velocity
<bonds
breaking re-forming
surface area speed
Ex: Same wooden box
__________ Ff,k
a/ On different sides: b/ At different speeds:
Ff,k is roughly independent of
a/ ________________ , and b/ _______________ .
v v
__________ Ff,ksame same
max.
v v
Ff
Ff,max
Static___ canbe ____valuein here
bonds______
Ff,k
_________ Friction ____________ Friction
Static _______ > _______
A graphical view of static vs. kinetic friction:
Static Kinetic
stre
tchi
ng
bond
s __
____
____
___
break bonds ____________and ______________
breakre-form
Ff
any
Ff,max Ff,k
In either static or kinetic friction, sliding frictiondepends on two factors:
1. The____________________________ – waxed skis on snow, rubber on dry concrete, etcUsually, smoother surfaces __________ friction
2. The force that________________________________ This force is the ____________ force: ______
FN = ______
FN
w
But press down on the block:
FN =_________
FN
w
F
F
Block on a table:
LESS
nature of the surfaces
presses the surfaces togethernormal FN
w w + F
These 2 factors are summed up in the equation:
Ff =
is called the _________________ of friction,
and is the Greek letter “______. ” It:
- describes effect of surface ______________ on Ff
- is usually _____________ for rougher surfaces - has ____ units: = _______ units _________ - has different values for: 1/ maximum static friction: _____ 2/ kinetic friction : and with ____ > ____ .
FN
coefficientmu
roughnessbigger
cancelno
k
s
s k
Ff/FN
k s
For maximum static Ff,max, use: Ff = FN
For kinetic Ff,k, use: Ff = ____FN
Notice:
1.This should say
___________ Static
2. k ___ s
From the PhysRT, page ___ :1
Maximum
<
k
s
Ex: A 12,000-N crate is on waxed skis on snow. Find the force needed to…1/ start it moving, and2/ keep it moving at constant velocity
w = 12,000 N
FN = 12,000 N
1/ max. static: Ff = FN sFN
2/ kinetic: Ff = FN kFN
How would your answers change if another 2000 N were added to the sled?
=
=
=
=
(12,000 N) 1700 N
(12,000 N)600 N=
=
Open a textbook to thepage 145.
Do: #1,2 and 3
no pullno friction
no stretchingof bonds
slight pullsome Ff
some stretching
maximumstretching
just before itbreaks free: Ff,max