Wheels 1 Wheels 2 Observations about Wheels Observations about Wheels � Friction makes wheel Friction makes wheel- -less objects skid to a stop less objects skid to a stop Wheels Wheels � Friction wastes energy Friction wastes energy � Wheels mitigate the effects of friction Wheels mitigate the effects of friction � Wheels can also propel vehicles Wheels can also propel vehicles Wheels 3 Wheels 4 5 Questions about Wheels 5 Questions about Wheels Question 1 Question 1 � Why does a wagon need wheels? Why does a wagon need wheels? � Why does a wagon need wheels? Why does a wagon need wheels? � Why do sleds seem to “break free” and then Why do sleds seem to “break free” and then � Why do sleds work well only on snow or ice? Why do sleds work well only on snow or ice? slide easily when you shove them hard enough? slide easily when you shove them hard enough? � 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 ? ? � 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 Frictional Forces Frictional Forces The Two Types of Friction The Two Types of Friction � A frictional force A frictional force � Static Friction Static Friction � opposes relative sliding motion of two surfaces opposes relative sliding motion of two surfaces � Acts to prevent objects from Acts to prevent objects from starting starting to slide to slide � points along the surfaces points along the surfaces � Forces can range from zero to an upper limit Forces can range from zero to an upper limit � 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 � Sliding Friction Sliding Friction Slidi Slidi F i i F i i � rd law pairs: Frictional forces always come in 3 rd � Frictional forces always come in 3 law pairs: � Acts to stop objects that are Acts to stop objects that are already already sliding sliding � Pavement’s frictional force pushes cart backward Pavement’s frictional force pushes cart backward � Forces have a fixed magnitude Forces have a fixed magnitude � Cart’s frictional force pushes pavement forward Cart’s frictional force pushes pavement forward 1
Wheels 7 Wheels 8 Question 2 Question 2 Frictional Forces Frictional Forces � Why do sleds seem to “break free” and then Why do sleds seem to “break free” and then � Frictional forces increase when you: Frictional forces increase when you: slide easily when you shove them hard enough? slide easily when you shove them hard enough? � push the surfaces more tightly together push the surfaces more tightly together � roughen the surfaces roughen the surfaces � Peak static force is greater than sliding force P Peak static force is greater than sliding force P k k i f i f i i h h lidi lidi f f � Surface features can interpenetrate better Surface features can interpenetrate better � Friction force drops when sliding begins Friction force drops when sliding begins Wheels 9 Wheels 10 Sleds and Friction Sleds and Friction Question 3 Question 3 � A stationary sled A stationary sled � What happens to energy as a sled skids to rest? What happens to energy as a sled skids to rest? � � experiences static friction experiences static friction � won’t start moving until you pull very hard won’t start moving until you pull very hard � A moving sled A moving sled A A i i l d l d � experiences sliding friction experiences sliding friction � 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 Friction, Energy, and Wear Friction, Energy, and Wear The Many Forms of Energy The Many Forms of Energy � Static friction Static friction � Kinetic: energy of motion Kinetic: energy of motion � � Both surfaces travel the same distance (often zero) Both surfaces travel the same distance (often zero) � Potential: stored in forces between objects Potential: stored in forces between objects � No work “disappears” and there is no wear No work “disappears” and there is no wear � Gravitational Gravitational � Elastic Elastic � � Sliding friction Sliding friction Slidi Slidi f i i f i i � Magnetic M Magnetic M i i � Electric Electric El El i i � � Electrochemical Electrochemical � Chemical Chemical � The two surfaces travel different distances The two surfaces travel different distances � � Nuclear Nuclear � Some work “disappears” and becomes thermal energy Some work “disappears” and becomes thermal energy � Thermal energy: the same forms of energy, but Thermal energy: the same forms of energy, but � The surfaces experience wear The surfaces experience wear divided up into countless tiny fragments divided up into countless tiny fragments � A sliding box turns energy into thermal energy A sliding box turns energy into thermal energy 2
Wheels 13 Wheels 14 Energy and Order Energy and Order Question 4 Question 4 � A portion of energy can be A portion of energy can be � How do wheels help a wagon coast? How do wheels help a wagon coast? � Organized Organized – – ordered energy (e.g. work) ordered energy (e.g. work) � Fragmented Fragmented – – disordered energy (e.g. thermal energy) disordered energy (e.g. thermal energy) � Turning ordered energy into disordered energy � Turning ordered energy into disordered energy Turning ordered energy into disordered energy Turning ordered energy into disordered energy � is easy to do is easy to do � is statistically likely is statistically likely � Turning disordered energy into ordered energy Turning disordered energy into ordered energy � is hard to do is hard to do � is statistically unlikely (it’s effectively impossible) is statistically unlikely (it’s effectively impossible) Wheels 15 Wheels 16 Rollers Rollers Wheels Wheels � Eliminate Eliminate sliding sliding friction at roadway friction at roadway � Eliminate Eliminate sliding sliding friction friction � at roadway at roadway � Convenient because they don’t pop out Convenient because they don’t pop out � Allow Allow static static friction to exert torques on wheels friction to exert torques on wheels � Are inconvenient because Are inconvenient because � and forces on vehicle and forces on vehicle and forces on vehicle and forces on vehicle they keep popping out they keep popping out they keep popping o t they keep popping o t � Wheel hubs still have Wheel hubs still have sliding sliding friction friction from under the object from under the object Wheels 17 Wheels 18 Bearings Bearings Practical Wheels Practical Wheels � Eliminate Eliminate sliding sliding � Free wheels are turned Free wheels are turned friction in wheel hub friction in wheel hub by the vehicle’s motion by the vehicle’s motion � Behave like Behave like � Powered wheels propel Powered wheels propel � automatically recycling automatically recycling a tomatically recycling a tomatically recycling the vehicle as they t rn the vehicle as they turn. the vehicle as they turn. the vehicle as they t rn rollers rollers 3
Wheels 19 Wheels 20 Question 5 Question 5 Wheels and Kinetic Energy Wheels and Kinetic Energy � What energy does a wheel have? What energy does a wheel have? � A moving wheel has kinetic energy: A moving wheel has kinetic energy: kinetic energy = ½ · mass · speed 2 kinetic energy = ½ · mass · speed 2 � A spinning wheel has kinetic energy: A spinning wheel has kinetic energy: kinetic energy = ½ · rotational mass · ki ki kinetic energy = ½ · rotational mass · ang i i ½ ½ i i l l ang. speed . speed 2 d 2 � A moving and spinning wheel has both A moving and spinning wheel has both � Both kinetic energies are transferred via work Both kinetic energies are transferred via work Wheels 21 Summary about Wheels Summary about Wheels � Sliding friction wastes energy Sliding friction wastes energy � Wheels eliminate sliding friction Wheels eliminate sliding friction � A vehicle with wheels coasts well A vehicle with wheels coasts well � Free wheels are turned by static friction F Free wheels are turned by static friction F h h l l d b d b i f i i i f i i � Powered wheels use static friction to propel car Powered wheels use static friction to propel car 4
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