Wheels 1 Wheels Wheels 2 Introductory Question n

Wheels 1 Wheels

Wheels 2 Introductory Question n The light turns green and you’re in a hurry. Will your car accelerate faster if you skid your wheels and “burn rubber” or if you just barely avoid skidding your wheels? A. Skid your wheels Barely avoid skidding B.

Wheels 3 Observations about Wheels Friction makes wheel-less objects skid to a stop n Friction wastes energy n Wheels mitigate the effects of friction n Wheels can also propel vehicles n

Wheels 4 4 Questions about Wheels Why does a wagon need wheels? n Why do sleds seem to “break free” and then slide easily when you shove them hard enough? n What happens to energy as a sled skids to rest? n How do wheels help a wagon coast? n What type of energy does a wheel have? n

Wheels 5 Question 1 Why does a wagon need wheels? n Why do sleds work well only on snow or ice? n

Wheels 6 Frictional Forces n A frictional force opposes relative sliding motion of two surfaces n points along the surfaces n acts to bring the two surfaces to one velocity n n Frictional forces always come in 3 rd law pairs: Pavement’s frictional force pushes cart backward n Cart’s frictional force pushes pavement forward n

Wheels 7 The Two Types of Friction n Static Friction Acts to prevent objects from starting to slide n Forces can range from zero to an upper limit n n Sliding Friction Acts to stop objects that are already sliding n Forces have a fixed magnitude n

Wheels 8 Question 2 n Why do sleds seem to “break free” and then slide easily when you shove them hard enough?

Wheels 9 Frictional Forces n Increase when you: push the surfaces more tightly together n roughen the surfaces n n Peak static force greater than sliding force Surface features can interpenetrate better n Friction force drops when sliding begins n

Wheels 10 Sleds and Friction n A stationary sled experiences static friction n won’t start moving until you pull very hard n n A moving sled experiences sliding friction n needs to be pulled or it will slow down and stop n experiences wear as it skids along the pavement n

Wheels 11 Introductory Question (revisited) n The light turns green and you’re in a hurry. Will your car accelerate faster if you skid your wheels and “burn rubber” or if you just barely avoid skidding your wheels? A. Skid your wheels Barely avoid skidding B.

Wheels 12 Question 3 n What happens to energy as a sled skids to rest?

Wheels 13 Friction, Energy, and Wear n Static friction (distance traveled is zero) n n No work and no wear Sliding friction (distance traveled is nonzero) Work and wear n Work is turned into thermal energy n n A sliding box turns energy into thermal energy

Wheels 14 The Many Forms of Energy Kinetic: energy of motion n Potential: stored in forces between objects n n n Gravitational Magnetic Electrochemical Nuclear n n n Elastic Electric Chemical Thermal energy: the same forms of energy, but divided up into countless tiny fragments

Wheels 15 Energy and Order n A portion of energy can be Organized – ordered energy (e. g. work) n Fragmented – disordered energy (e. g. thermal energy) n n Turning ordered energy into disordered energy is easy to do n is statistically likely n n Turning disordered energy into ordered energy is hard to do n is statistically unlikely (it’s effectively impossible) n

Wheels 16 Question 4 n How do wheels help a wagon coast?

Wheels 17 Rollers Eliminate sliding friction at roadway n Are inconvenient because they keep popping out from under the object n

Wheels 18 Wheels Eliminate sliding friction at roadway n Convenient because they don’t pop out n Allow static friction to exert torques on wheels and forces on vehicle n Wheel hubs still have sliding friction n

Wheels 19 Bearings Eliminate sliding friction in wheel hub n Behave like automatically recycling rollers n

Wheels 20 Question 5 n What type of energy does a wheel have?

Wheels 21 Wheels and Kinetic Energy n A moving wheel has kinetic energy: kinetic energy = ½ · mass · speed 2 n A spinning wheel has kinetic energy: kinetic energy = ½ · rotational mass · ang. speed 2 n Both kinetic energies are transferred via work

Wheels 22 Summary about Wheels n Sliding friction wastes energy Wheels eliminate sliding friction n A vehicle with wheels coasts well n Free wheels are turned by static friction n Powered wheels use static friction to propel car n