2.2 Wheels2.2 Wheels

Ideas for todayIdeas for today• Friction• Wheels and bearings• Kinetic Energy• Power

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?

What is friction?What is friction?

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.

How do you avoid a skid?

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)

Making, designing, and repairing bearings is a big business!

BearingsBearings

Rolling resistanceRolling resistance

Friction within the tire generates thermal energy

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

1 horsepower = 746 Watts = 746 joules / sec

Power is measured in WattsPower is measured in Watts

• Remember that there are all kinds of energy: elastic, chemical, gravitational potential, thermal, kinetic…

• The total of all the energy is conserved!

See you next class!

For next class: Read Section 2.3