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Friction The force of_________________, F f: opposes 1. Ff = A ____________ force that ____________ contact breaking and motion. It is caused by the __________________ re-forming of electromagnetic bonds between ______________________________________ its environment an object and _________________________. heating when 2. Friction usually results in __________ molecules in motion energy is transferred to ______________ the object and its environment. This results in a phase change increase temperature _____________ or a _________________, melting such as _____________. Ex: car skidding on road kinetic energy ________ tire heats up ______ road and _________ molecules Three basic types of friction: fluid A._____________ friction– due to collisions with fluid molecules; increases with speed water or air resistance Ex: _________________________________ rolling B._____________ friction –reduces amount of surface contact friction because there is less __________________ ball bearings Ex: ___________________ flat C. ______________ friction – when two _______ sliding surfaces slide past each other; what we _____________ are going to learn most about. most least sliding ___________ rolling fluid force __________ __________ force Sliding Friction: All About ___________ A 10-N object at rest: Forces acting on it: FN = 10 N 10 N 0 Fnet =____ w = 10 N Now pull it with a 2 N force but it doesn’t move: 10 N pull Fp = 2N 0 Fnet =____ FN = 10 N Ff = 2N Fp = 2 N w = 10 N Pull it with a 4 N force and it still doesn’t move: FN = 10 N Ff = 4 N pull = 4 N 10 N 0 Fnet =____ w = 10 N When you increase the force accelerates to 6.0 N, it _______________: 10 N pull = 6 N Fnet ___ ≠ 0 Fp = 4 N Ff < 6N FN = 10 N Fp = 6 N w = 10 N maximum __________ Ff,max static _______ friction = _____ 5.0 N Once it starts moving, you only need a _______ constant velocity pull to keep it moving at _____________________: 10 N FN = 10 N pull = 5 N Ff = 5 N 0 Fnet = ____ Fp = 5 N w = 10 N kinetic __________ Ff , k (moving) friction = _____ __________ kinetic __________ friction Ff,k < maximum ___________ static Ff,max Note: Static 1._____________ friction, Ff at rest friction when body is_______________ any amount up to _______________ maximum can be _____ static friction Ff,max 2. Maximum static friction, __________ breaks free friction just before the object ________________ electromagnetic During static friction, the _________________ bonds between the object and the molecular _________ stretching surface are ________________ microscopically, at more and more As the first a little, then __________________. breaking bonds reach the ________________ point, maximum static _______________________ friction occurs. 3. _________ Kinetic friction, Ff,k is the friction that occurs constant velocity when an object moves at ____________________ . always ___ < the__________ max. static friction: Ff,max bonds are As it slides, the surface _________ breaking and________________ re-forming constantly ____________ Ex: Same wooden box a/ On different sides: v v same F __________ f,k b/ At different speeds: v v same __________ Ff,k Ff,k is roughly independent of surface area , and a/ ________________ speed b/ _______________ . A graphical view of static vs. kinetic friction: Ff bonds break ______ Ff,max Static Ff can ___ any be ____ value in here break bonds ____________ re-form and ______________ Ff,k Static Friction _________ Ff,k Ff,max > _______ Static _______ Kinetic ____________ Friction In either static or kinetic friction, sliding friction depends on two factors: nature of the surfaces 1. The____________________________ – waxed skis on snow, rubber on dry concrete, etc LESS Usually, smoother surfaces __________ friction presses the surfaces together 2. The force that________________________________ FN normal This force is the ____________ force: ______ But press down on the block: Block on a table: FN F w w FN = ______ FN w+F FN =_________ w F These 2 factors are summed up in the equation: Ff = mFN coefficient m is called the _________________ of friction, mu ” It: and is the Greek letter “______. roughness on Ff - describes effect of surface ______________ bigger - is usually _____________ for rougher surfaces Ff/FN units _________ cancel - has ____ no units: m = _______ - has different values for: ms 1/ maximum static friction: _____ mk 2/ kinetic friction : _____, ms > ____ mk . and with ____ 1 : From the PhysRT, page ___ Notice: 1.This should say ___________ Maximum Static 2. mk mk ___ < ms ms ms FN For maximum static Ff,max, use: Ff = ____ mk N For kinetic Ff,k, use: Ff = ____F 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 Ff = mFN = mkFN 2/ kinetic: = = (0.05) (12,000 N) 600 N How would your answers change if another 2000 N were added to the sled?