Wheels 1 Wheels 2 Introductory Question Introductory Question The light turns green and you’re in a hurry. The light turns green and you’re in a hurry. � Will your car accelerate faster if you skid your Will your car accelerate faster if you skid your Wheels Wheels wheels and “burn rubber” or if you just barely wheels and “burn rubber” or if you just barely avoid skidding your wheels? avoid skidding your wheels? avoid skidding your wheels? avoid skidding your wheels? Skid your wheels Skid your wheels A. A. Barely avoid skidding Barely avoid skidding B. B. Wheels 3 Wheels 4 Observations about Wheels Observations about Wheels 5 Questions about Wheels 5 Questions about Wheels � Friction makes wheel Friction makes wheel- -less objects skid to a stop less objects skid to a stop � Why does a wagon need wheels? Why does a wagon need wheels? � Friction wastes energy Friction wastes energy � Why do sleds seem to “break free” and then Why do sleds seem to “break free” and then � slide easily when you shove them hard enough? slide easily when you shove them hard enough? � Wheels mitigate the effects of friction Wheels mitigate the effects of friction � � What happens to energy as a sled skids to rest? What happens to energy as a sled skids to rest? Wh Wh h h l d kid l d kid ? ? � Wheels can also propel vehicles Wheels can also propel vehicles � How do wheels help a wagon coast? How do wheels help a wagon coast? � What energy does a wheel have? What energy does a wheel have? Wheels 5 Wheels 6 Question 1 Question 1 Frictional Forces Frictional Forces � Why does a wagon need wheels? Why does a wagon need wheels? � A frictional force A frictional force � � Why do sleds work well only on snow or ice? Why do sleds work well only on snow or ice? � opposes relative sliding motion of two surfaces opposes relative sliding motion of two surfaces � � points along the surfaces points along the surfaces � acts to bring the two surfaces to one velocity acts to bring the two surfaces to one velocity t t brin th t t t brin th t rf rf t t n n l l it it � rd law pairs: Frictional forces always come in 3 rd � Frictional forces always come in 3 law pairs: � Pavement’s frictional force pushes cart backward Pavement’s frictional force pushes cart backward � Cart’s frictional force pushes pavement forward Cart’s frictional force pushes pavement forward 1
Wheels 7 Wheels 8 The Two Types of Friction The Two Types of Friction Question 2 Question 2 � Static Friction Static Friction � Why do sleds seem to “break free” and then Why do sleds seem to “break free” and then � slide easily when you shove them hard enough? slide easily when you shove them hard enough? � Acts to prevent objects from Acts to prevent objects from starting starting to slide to slide � Forces can range from zero to an upper limit Forces can range from zero to an upper limit � Sliding Friction Sliding Friction Slidi Slidi F i i F i i � Acts to stop objects that are Acts to stop objects that are already already sliding sliding � Forces have a fixed magnitude Forces have a fixed magnitude Wheels 9 Wheels 10 Frictional Forces Frictional Forces Sleds and Friction Sleds and Friction � Frictional forces increase when you: Frictional forces increase when you: � A stationary sled A stationary sled � push the surfaces more tightly together push the surfaces more tightly together � experiences static friction experiences static friction � roughen the surfaces roughen the surfaces � won’t start moving until you pull very hard won’t start moving until you pull very hard � Peak static force is greater than sliding force Peak static force is greater than sliding force P P k k i f i f i i h h lidi lidi f f � A moving sled A moving sled A A i i l d l d � Surface features can interpenetrate better Surface features can interpenetrate better � experiences sliding friction experiences sliding friction � Friction force drops when sliding begins Friction force drops when sliding begins � needs to be pulled or it will slow down and stop needs to be pulled or it will slow down and stop � experiences wear as it skids along the pavement experiences wear as it skids along the pavement Wheels 11 Wheels 12 Introductory Question Introductory Question (revisited) Question 3 Question 3 (revisited) The light turns green and you’re in a hurry. The light turns green and you’re in a hurry. � What happens to energy as a sled skids to rest? What happens to energy as a sled skids to rest? � Will your car accelerate faster if you skid your Will your car accelerate faster if you skid your wheels and “burn rubber” or if you just barely wheels and “burn rubber” or if you just barely avoid skidding your wheels? avoid skidding your wheels? avoid skidding your wheels? avoid skidding your wheels? Skid your wheels Skid your wheels A. A. Barely avoid skidding Barely avoid skidding B. B. 2
Wheels 13 Wheels 14 Clicker Question Clicker Question Friction, Energy, and Wear Friction, Energy, and Wear � When a block skids to a stop on a motionless When a block skids to a stop on a motionless � Static friction Static friction desk, what work is done by those two objects? desk, what work is done by those two objects? � Both surfaces travel the same distance (often zero) Both surfaces travel the same distance (often zero) � No work “disappears” and there is no wear No work “disappears” and there is no wear � Sliding friction Sliding friction Slidi Slidi f i i f i i A. The desk does negative work on the block. The desk does negative work on the block. Th d k d Th d k d i i k k h bl h bl k k A. � The two surfaces travel different distances The two surfaces travel different distances B. The block does positive work on the desk. The block does positive work on the desk. B. � Some work “disappears” and becomes thermal energy Some work “disappears” and becomes thermal energy C. Both objects do positive work on one another. Both objects do positive work on one another. C. � The surfaces experience wear The surfaces experience wear D. Both objects do negative work on one another. Both objects do negative work on one another. D. � A sliding box turns energy into thermal energy A sliding box turns energy into thermal energy E. The block does positive work on the desk, the The block does positive work on the desk, the E. desk does negative work on the block. desk does negative work on the block. Wheels 15 Wheels 16 The Many Forms of Energy The Many Forms of Energy Energy and Order Energy and Order � A portion of energy can be A portion of energy can be � Kinetic: energy of motion Kinetic: energy of motion � Organized Organized – – ordered energy (e.g. work) ordered energy (e.g. work) � Potential: stored in forces between objects Potential: stored in forces between objects � Fragmented Fragmented – – disordered energy (e.g. thermal energy) disordered energy (e.g. thermal energy) � Gravitational Gravitational � Elastic Elastic � Turning ordered energy into disordered energy � Turning ordered energy into disordered energy Turning ordered energy into disordered energy Turning ordered energy into disordered energy � Magnetic M Magnetic M i i � Electric Electric El El i i � is easy to do is easy to do � Electrochemical Electrochemical � Chemical Chemical � is statistically likely is statistically likely � Nuclear Nuclear � Turning disordered energy into ordered energy Turning disordered energy into ordered energy � Thermal energy: the same forms of energy, but Thermal energy: the same forms of energy, but � is hard to do is hard to do divided up into countless tiny fragments divided up into countless tiny fragments � is statistically unlikely (it’s effectively impossible) is statistically unlikely (it’s effectively impossible) Wheels 17 Wheels 18 Question 4 Question 4 Rollers Rollers � How do wheels help a wagon coast? How do wheels help a wagon coast? � Eliminate Eliminate sliding sliding friction friction at roadway at roadway � Are inconvenient because Are inconvenient because they keep popping o t they keep popping out they keep popping o t they keep popping out from under the object from under the object 3
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