Wheels. Wheels 2. Wheels 4. Wheels 3. Wheels 5. Wheels 6

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1 Wheels 1 Wheels 2 Introductory Queston Wheels The lght turns green and you re n a hurry. Wll your car accelerate faster f you skd your wheels and burn rubber or f you just barely avod skddng your wheels? A. Skd your wheels B. Barely avod skddng Wheels 3 Observatons about Wheels Wheels 4 5 Questons about Wheels Frcton makes wheel-less less objects skd to a stop Frcton wastes energy Wheels mtgate the effects of frcton Wheels can also propel vehcles Why does a wagon need wheels? Why do sleds seem to break free and then slde easly when you shove them hard enough? What happens to energy as a sled skds to rest? How do wheels help a wagon coast? What energy does a wheel have? Wheels 5 Queston 1 Wheels 6 Frctonal Forces Why does a wagon need wheels? Why do sleds work well only on snow or ce? A frctonal force opposes relatve sldng moton of two surfaces ponts along the surfaces acts to brng the two surfaces to one velocty Frctonal forces always come n 3 rd law pars: Pavement s frctonal force pushes cart backward Cart s frctonal force pushes pavement forward 1

2 Wheels 7 The Two Types of Frcton Wheels 8 Queston 2 Statc Frcton Acts to prevent objects from startng to slde Forces can range from zero to an upper lmt Sldng Frcton Acts to stop objects that are already sldng Forces have a fxed magntude Why do sleds seem to break free and then slde easly when you shove them hard enough? Wheels 9 Frctonal Forces Wheels 10 Sleds and Frcton Frctonal forces ncrease when you: push the surfaces more tghtly together roughen the surfaces Peak statc force s greater than sldng force Surface features can nterpenetrate better Frcton force drops when sldng begns A statonary sled experences statc frcton won t start movng untl you pull very hard A movng sled experences sldng frcton needs to be pulled or t wll slow down and stop experences wear as t skds along the pavement Wheels 11 Introductory Queston (revsted) Wheels 12 Queston 3 The lght turns green and you re n a hurry. Wll your car accelerate faster f you skd your wheels and burn rubber or f you just barely avod skddng your wheels? What happens to energy as a sled skds to rest? A. Skd your wheels B. Barely avod skddng 2

3 Wheels 13 Clcker Queston Wheels 14 Frcton, Energy, and Wear When a block skds to a stop on a motonless desk, what work s done by those two objects? A. The desk does negatve work on the block. B. The block does postve work on the desk. C. Both objects do postve work on one another. D. Both objects do negatve work on one another. E. The block does postve work on the desk, the desk does negatve work on the block. Statc frcton Both surfaces travel the same dstance (often zero) No work dsappears and there s no wear Sldng frcton The two surfaces travel dfferent dstances Some work dsappears and becomes thermal energy The surfaces experence wear A sldng box turns energy nto thermal energy Wheels 15 The Many Forms of Energy Wheels 16 Energy and Order Knetc: energy of moton Potental: stored n forces between objects Gravtatonal Magnetc Electrochemcal Nuclear Elastc Electrc Chemcal Thermal energy: the same forms of energy, but dvded up nto countless tny fragments A porton of energy can be Organzed ordered energy (e.g. work) Fragmented dsordered energy (e.g. thermal energy) Turnng ordered energy nto dsordered energy s easy to do s statstcally lkely Turnng dsordered energy nto ordered energy s hard to do s statstcally unlkely (t s effectvely mpossble) Wheels 17 Queston 4 Wheels 18 Rollers How do wheels help a wagon coast? Elmnate sldng frcton at roadway Are nconvenent because they keep poppng out from under the object 3

4 Wheels 19 Clcker Queston Wheels 20 Wheels When you pull a wheeled cart forward and t s acceleratng forward, what frctonal force does the ground exert on the wheels as they roll? A. A sldng frctonal force that ponts backward. B. A statc frctonal force that ponts forward. C. A statc frctonal force that ponts backward. D. A sldng frctonal force that ponts forward. E. Zero frctonal force. Elmnate sldng frcton at roadway Convenent because they don t pop out Allow statc frcton to exert torques on wheels and forces on vehcle Wheel hubs stll have sldng frcton Wheels 21 Bearngs Wheels 22 Practcal Wheels Elmnate sldng frcton n wheel hub Behave lke automatcally recyclng rollers Free wheels are turned by the vehcle s moton Powered wheels propel the vehcle as they turn. Wheels 23 Queston 5 Wheels 24 Wheels and Knetc Energy What energy does a wheel have? A movng wheel has knetc energy: knetc energy = ½ mass speed 2 A spnnng wheel has knetc energy: knetc energy = ½ rotatonal mass ang. speed 2 A movng and spnnng wheel has both Both knetc energes are transferred va work 4

5 Wheels 25 Summary about Wheels Sldng frcton wastes energy Wheels elmnate sldng frcton A vehcle wth wheels coasts well Free wheels are turned by statc frcton Powered wheels use statc frcton to propel car 5