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

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

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 =____

≠ 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

< 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 <

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

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

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 _______ > _______

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 =_________

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

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

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?