Re: Truck stopping distances?

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

According to our simple frictional models, what would be the same for truck
and car would be the stopping distance once the wheels are frozen in place
and the car/truck skids to a stop.  Of course we don't want to put either
into a skid, so the rotation of the wheels is slowed in a way so as NOT to
exceed the maximum value of static friction.  That is done through the
brakes.  You need to concentrate therefore on the brakes, not the tire/road
contact.  The brake force is not a function of the weight of the vehicles
but rather the size, composition, and operation of the brakes.  The
difference in stopping distance between trucks and cars (as between
different makes of cars) is tied up with the brake design.  In the case of
trucks, I also suspect the longer distances are due to a compromise between
safety and brake life--that is, while brakes could be designed to stop the
trucks in a shorter distance, they would tend to wear out much faster.
Ultimately the brakes must stop the wheel rotation, and the greater momentum
of the truck will require larger brake forces.

This is my quick take on the question, others will probably have more
details.

Rick

**********************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

FREE PHYSICS INSTRUCTIONAL SOFTWARE
www.saintmarys.edu/~rtarara/software.html
PC and MAC software
NEW! SIMLAB2001--DYNAMIC CARTS now available.
CD-ROMs now available
******************************************************


----- Original Message -----
From: "Scott Goelzer" <sgoelzer@EARTHLINK.NET>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, March 01, 2002 7:59 AM
Subject: Truck stopping distances?


> Why does it take a truck further to stop than a car? Data on many web
sites
> gives about a 50% increase in stopping distances.
>
> BUT
>
> HS physics friction problems typically ask students to calculate the
> stopping distance of a vehicle with speed x and COF y. This shows that the
> mass of the object is cancelled when finding the acceleration and
therefore
> should be independent of the stopping distance.
>
> Same tires, same pavement.
> What is different about the braking systems?
>
> Web searches only reveal that the distances are greater. No mention of
why -
> besides the fact that trucks are heavier - useless.
>
> Scott
>
>
>
> *****************************
> S.Goelzer
> Physics Teacher
> Coe - Brown Northwood Academy
> Northwood NH 03261
> 603-942-5531 ext43
> sgoelzer@coebrownacademy.com
> *****************************