[Phys-L] Re: Stopping Distance

[rtarara at SAINTMARYS.EDU (Rick Tarara)]

----- Original Message -----
From: "John Mallinckrodt" <ajm@CSUPOMONA.EDU>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Monday, October 31, 2005 7:57 AM
Subject: Re: Stopping Distance


> >  >In yesterday's Parade magazine there was a statement that large
> >  >trucks require more distance to stop than do light trucks and cars,
> >  >as much as 50% percent more distance.  Does anyone know the
> >  >explanation as to why this is?
> >  >
> >  >Gene
> >
> > The ratio of max braking force to max laden weight is less
> > favorable than for passenger cars.
>
> Knowing Gene pretty well, I suspect he had sussed that much out.  I
> would imagine that his question is really more like, "Why should that
> be, considering the fact that the coefficients of friction between
> the tires and the road ought to be very similar in both cases?"  I
> don't know the answer either, but I suspect it's more a result of
> simple safety considerations.  One doesn't want to be handling a
> loaded semi as close to the brake locking point as one might be
> willing to risk in a passenger car.

I think you really have to look to the brakes and pads--not at the tires and
road.  A fully loaded truck can have 30-40 times more KE at a given speed
than a car.  While the brakes are bigger, more of them, and presumably
produce larger forces, one still has to scrub off the KE into heat.  [One of
the more dramatic 'physics' things one can see on TV is when they have a
camera mounted under a NASCAR vehicle looking at the brake rotors.  On some
tracks, as they slow into a turn, the rotor gets hot enough to start
glowing!]

As with most things, I suspect there have been cost/benefit calculations
made in terms of the size and strength of these brakes.  Add into this that
running the brakes until they are completely worn and running on tires with
minimum tread will increase profits.  [Current diesel fuel prices will
encourage more 'to the edge' truck maintenance.]  This is one of the reasons
for the truck run-off lanes in the mountains--imminent failure of overly
worn brakes.  ;-)

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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