2.2 wheels. ideas for today friction wheels and bearings kinetic energy power
TRANSCRIPT
Observations About Observations About WheelsWheels
• 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?
Sliding friction is a force that Sliding friction is a force that opposes relative motionopposes relative motion
Frictional force on bottle
Frictional force on table
Sliding friction tends to convert kinetic energy into “thermal” energy
Simulation Courtesy of PHET
Cork Cork popperpopper
Types of FrictionTypes 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 Ramp with blocksblocks
Frictional ForcesFrictional 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 FrictionStatic/Sliding Friction
WheelsWheels
• Eliminate sliding friction at roadway
• Use static friction to propel vehicle
• Wheel hubs integrate bearings to eliminate sliding friction at the axel
Bicycle WheelBicycle Wheel
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
v
Static friction
The relative velocity is zero right at the bottom if the wheel is not slipping!
Wfriction=F║ d = friction × 0 = 0
This is the torque that makes the wheel turn!
Clicker questionClicker questionYou 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.
This static friction is what makesThis 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.and that’s another reason ABS was developed.
BearingsBearings
• Eliminate sliding friction in wheel hub
• Behave like wheels (roll without slipping)
Clicker questionClicker 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
PowerPower
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
• Remember that there are all kinds of energy: elastic, chemical, gravitational potential, thermal, kinetic…
• The total of all the energy is conserved!