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Re: [Phys-l] Work done by Static Friction



At 01:37 PM 3/17/2007, Bob, you wrote:

Easily contentious! I am uneasy about your chosen path, which probably
was not meant literally. The energy from the engine appears not in the
wheels, but in the entire vehicle. There are hundreds of internal
action-reation pairs that have work associated with them that appears
as KE of all the various parts of the vehicle, just as in pushing ourselves
away from a wall the work from the internal contraction of our muscles
ends up as our gross kinetic energy.

Bob at PC


I am not personally thrilled when the living body figures in energy/work
illustrations. I know perfectly well, that energy is consumed to hold a
person standing still (and not falling over) and for me this muddies
the idea of converting a muscular contraction to kinetic energy that
pushing away from a wall entails.

And though Bob is happy to visualize hundreds of action/reaction pairs
in a moving truck, I prefer to think like this:
the person who designed this vehicle took care to provide a path for the
engine torque, and the reaction to that torque all the way to the drive
wheels. I know he will have specified hub bearings at the drive axle,
leaf springs and shackles or suspension of whatever kind of sufficient
strength to allow the force which pushes the axle forward to accelerate the
entire laden mass with a reasonable margin of safety.

So for me, associating a kinetic energy with a complete vehicle mass is
OK and for me visualizing the force that succeeds in pushing that mass to
increasing speed is OK I can easily agree that the whole mass acquires
the kinetic energy of its speed, and acquires potential energy when climbing
up hill.
Yes of course: but I am focusing rather, on where that force comes from.
I see that force as originating at the driven wheels' hubs.


Brian Whatcott Altus OK Eureka!