1. 2.2 Wheels

2. Ideas for today Friction Wheels and bearings Kinetic Energy Power

3. Observations About Wheels Without wheels, objects slide to a stop Friction is responsible for stopping Friction seems to make energy disappear Wheels eliminate friction, or so it seems Wheels can also propel vehicles, but how?

4. What is friction?

5. Sliding friction is a force that opposes relative motion Frictional force on bottle Frictional force on table

6. Sliding friction tends to convert kinetic energy into “thermal” energy Simulation Courtesy of PHET Cork popper

7. Types of Friction Sliding FrictionSliding Friction –Acts to stop objects that are already sliding –Forces have fixed magnitudes Static FrictionStatic Friction –Acts to prevent objects from starting to slide –Forces can vary from zero to an upper limit Ramp with blocks

8. Frictional Forces Increase when you: –push the surfaces more tightly together –roughen the surfaces Peak static force is greater than sliding force: –Surface features can interpenetrate better Friction force drops when sliding begins Static/Sliding Friction

9. Wheels Eliminate sliding friction at roadway Use static friction to propel vehicle Wheel hubs integrate bearings to eliminate sliding friction at the axel Bicycle Wheel

10. Wheels use static friction to control motion and eliminate the production of thermal energy Static friction: force that prevents two surfaces from starting to slip

11. v Static friction The relative velocity is zero right at the bottom if the wheel is not slipping! W friction =F ║ d = friction × 0 = 0 This is the torque that makes the wheel turn!

12. Clicker question You are driving at 40 mph, when you suddenly step on the gas pedal. The care accelerates forward because A) there is very little friction between the tires and the road. B) there is static friction between the tires and the road. C) there is sliding friction between the tires and the road. D) the torque from the engine overcomes the frictional forces.

13. How do you avoid a skid?

14. This static friction is what makes it possible to steer a car… it possible to steer a car… …and that’s another reason ABS was developed.

15. Bearings Eliminate sliding friction in wheel hub Behave like wheels (roll without slipping)

16. Making, designing, and repairing bearings is a big business! Bearings

17. Rolling resistance Friction within the tire generates thermal energy

18. Clicker question You are pedaling your bike up a very big hill. If you account for friction, how can you minimize the energy that you must expend to get to the top? (A) use a bike with fat tires (inflated to lower pressure), so that the tires act like shock absorbers (B) use a bike with skinny tires (inflated to higher pressure), so that the tires are rigid and do not bend (C) it always takes the same amount of energy to reach the top of the hill, even with friction

19. Power Power = work / time With more power, more work can be done (and more energy transferred) in the same amount of time The rate at which energy is transferred

20. 1 horsepower = 746 Watts = 746 joules / sec Power is measured in Watts

21. Remember that there are all kinds of energy: elastic, chemical, gravitational potential, thermal, kinetic… The total of all the energy is conserved!

22. See you next class! For next class: Read Section 2.3