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Re: positive and negative work



At 09:45 AM 11/11/01 -0500, Carl Mungan wrote:
Brian Whatcott wrote:

> We use Professor Leigh's favorite model: the car rolling on an incline.
> Let us suppose that Michael is sitting in this car, which has started
to roll
> backwards, downhill. He wants to proceed slowly astern, so he applies a
little
> throttle while he is in a forward gear. This prevents the car from
> careering away.
>
> The work done by the car, we say, is negative. There is no discussion,
> no debate.
> This is the convention.

I disagree. The car exerts a force on the road surface, which is
stationary, and the tire is rolling so the frictional force is static, not
kinetic. It seem to me that the car is not doing any work on the road
surface. Is there anything else the car is doing work on?

Gene

--
Eugene P. Mosca
301 Constitution Blvd.
Kutztown, PA 19530
(610) 683-3597
emosca@ptd.net


Hmmmm...we agree that a car would accelerate down an incline,
if a force acting 'uphill' were not holding its speed steady.
We see that this retarding force varies with the throttle so we are
confident that the car is supplying this retarding force.
We see that this force acts opposite the motion, so we say by convention
the work is negative. But this is another way of saying that if the car's
work on the road is negative, the road's work on the car is positive.

Carl's model does not seem to offer a mechanism. Would it be clearer if
the tires were locked, and the car were slowly skidding down hill?

Perhaps not. What if the car were stationary, and the road were a
treadmill running uphill? The treadmill is then applying a force in the
direction
of its motion at a given velocity. This seems to describe a rate of
[positive] working.

What did I miss? :-)


Brian Whatcott
Altus OK Eureka!