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