Without wheels, a wagon would experience sliding friction as it moved across the
sidewalk. That sliding friction would grind up the
wagon's ordered energy into thermal energy.
And wear out both the bottom of the wagon and the top of the sidewalk.
Adding wheels doesn't eliminate all friction.
But it does eliminate sliding friction between the wagon and the sidewalk.
In normal situation, the wagon's wheels and the sidewalk experience only static
friction, this touch and release stuff. And static friction doesn't waste energy.
Although sliding friction still occurs between each wheel's hub and axle.
They don't waste much energy. Because the hub and axle don't move very
far relative to one another. And so the work done against sliding
friction in that hub and axle. Is quite small.
Even that, sliding friction, can be eliminated by inserting, roller bearings
or ball bearings between the hub and axle.
[SOUND] When you pull a wagon forward, the sidewalk exerts static frictional
forces on the wagon's free wheels. And those static frictional forces
produce torques of the wheels that 'cuz them to undergo angular acceleration and
begin to turn. In contrast, when you pedal a bicycle,
you are exerting a torque on that powered rear wheel.
In causing the sidewalk to exert a forward static frictional force on that
powered wheel that propels the entire bicycle forward.
Once the wagon or bicycle is moving and it's wheels are rotating and translating.
Those wheels have kinetic energy in both their translational motion.
And in their rotational motion. So, they have more than their fair share
of kinetic energy. In the context of wheels that we've
examined, frictional forces and their effects on motion.
We see how[UNKNOWN] energy can be ground into thermal energy.
And we've developed strategies to minimize the waste of energy by way of
static friction. We'll use those concepts often now as we
continue to study how things work.
Louis A. BloomfieldProfessor of Physics
