Re: [Phys-l] truck mileage

[Jack Uretsky <jlu@hep.anl.gov>]

Tichard misses the essential point, which I, apparently. did not make 
clearly:  There are scenarios where the added weight of the cows is an 
insignificant factor im the mpg.  One cannot, without more, therefore, 
dismiss the quoted report as false.
				Regards,
					Jack


On Wed, 7 May 2008, Richard Grandy wrote:

> That is assuming steady speed on level ground--accelerating from a
> stop or going up hill would seem more costly with the cows.
>
> Richard Grandy
> (in Houston, where it is all level ground)
>
> > Hi all-
> >  	Of course all such data need to be taken with a few grains of
> > salt, and driving habits can play a major role in the result.  But,
> > consider the physics of a truck pulling a trailer at constant speed on a
> > level road.  Power required be the presence of the trailer is that
> > dissipated by the rolling friction of the trailer wheels.  How much does
> > that friction increase when a load is placed obn the trailer?  I don't
> > have much feel for the numbers, but it must be a small fraction of the
> > added load.   The gal/mi is proportional to the friction force, which is
> > probably proportional to some power of the speed.  My conclusion is that
> > in all likelihood, a small decrease in the speed could compensate for the
> > added friction due to the increased load.
> >  			Regards,
> >  				Jack
> >
> >
> >
> > On Wed, 7 May 2008, Bill Nettles wrote:
> >
> > >  I agree the data are questionable
> > >
> > >  OR
> > >  the truck driver doesn't go as fast when he has 10000# of cows :)
> > >
> > >  Is there a baseline gas mileage with a nonlinear increase
> > > (arctangent) as the weight decreases? Gas mileages are notoriously
> > > non-linear for most variables.
> > >
> > > > > >  Kathy Daniel <kcdohs@comcast.net> 5/7/2008 4:02 pm >>>
> > >  I have quite an interesting problem that I cannot explain...
> > >
> > >  Our Ag teacher set up this scenario for me:
> > >
> > >  one 5000# truck gets about 18 miles per gallon
> > >  same 5000# truck pulling a 4000# trailer gets 8 miles per gallon
> > >  same 5000# truck pulling a 4000# trailer + 10000# of cows ALSO  gets 8
> > >  miles per gallon.
> > >
> > >  Any ideas why the empty and the full trailer both get the same
> > >  mileage?  This same pattern is also present in a different trailer and
> > >  truck combination.
> > >
> > >  thanks,
> > >  Kathy
> > >
> > >
> > >  Kathy Daniel
> > >  Oakland High School
> > >  Murfreesboro, TN  37130
> > >  kcdohs@comcast.net
> > >
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--
"Trust me.  I have a lot of experience at this."
		General Custer's unremembered message to his men,
		just before leading them into the Little Big Horn Valley