The force of_________________, F f : 1. F f = 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: car skidding on road ________ energy heats up ______ and _________ molecules kinetic tire road contact breaking and re-forming of electromagnetic bonds its 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 least force  __________  ___________  __________  force ball bearings flat surfaces sliding water or air resistance sliding rolling fluid surface contact

A 10-N object at rest: 10 N Forces acting on it: w = 10 N F N = 10 N Now pull it with a 2 N force but it doesn’t move: 10 N w = 10 N F N = 10 N pull F p = 2N F f = 2N F net =____ All About ___________ Friction: Sliding 0 0

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

Once it starts moving, you only need a _______ pull to keep it moving at _____________________: 10 N w = 10 N F N = 10 N pull = 5 N F f = 5 N F net = ____ F p = 5 N Ff,kFf,k ___________ static F f,max __________ friction F f, k < 5.0 N constant velocity 0 __________ __________ friction = _____ kinetic (moving) kinetic maximum

Note: 1._____________ friction, F f  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 rest Static any maximum F f,max breaks free electromagnetic stretching bonds more and more breaking maximum static

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

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

In either static or kinetic friction, sliding friction depends on two factors: 1. The____________________________ – waxed skis on snow, rubber on dry concrete, etc Usually, smoother surfaces  __________ friction 2. The force that________________________________  This force is the ____________ force: ______ F N = ______ FNFN w But press down on the block: F N =_________ FNFN w F F Block on a table: LESS nature of the surfaces presses the surfaces together normal FNFN w w + F

These 2 factors are summed up in the equation: F f =   is called the _________________ of friction, and is the Greek letter “______. ” It: - describes effect of surface ______________ on F f - is usually _____________ for rougher surfaces - has ____ units:  = _______  units _________ - has different values for: 1/ maximum static friction: _____ 2/ kinetic friction :   and with ____ > ____. FNFN coefficient mu roughness bigger cancel no kk ss ss kk F f /F N

kk ss For maximum static F f,max, use: F f =  F N For kinetic F f,k, use: F f = ____F N Notice: 1.This should say ___________ Static 2.  k ___  s From the PhysRT, page ___ : 1 Maximum < kk ss

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 w = 12,000 N F N = 12,000 N 1/ max. static: F f =  F N sFNsFN  2/ kinetic: F f =  F N kFN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 the page 145. Do: #1,2 and 3

no pull no friction no stretching of bonds slight pull some F f some stretching maximum stretching just before it breaks free: F f,max