[Phys-L] Re: Stopping Distance

[haar at PHYSICS.ARIZONA.EDU (Roger Haar)]

Dear Gene,

        I assume by now that it is appearent that the
cause of the greater stopping distance is
complex.  So far I have seen the following valid
reasons:

1.  Breakdown of the simple law:
     friction_force = mu times normal_force.
     This "law" just does not hold for rubber on
     asphalt as any drag racer will tell you..
2. Different rubber in the tires and higher tire
     inflation pressure.
     The reduced contact area per weight
associated
     with higher tire pressure relates to item 1
above.
3. Brakes not able to dissipate heat fast enough.
4. Wheels without brakes.

        Let me add one more factor.  Stability.  Locking
the brakes on a car can be bad bussiness, but is
relatively
quickly corrected.  If the brakes on an 18 wheeler
lock,
the trailer can start to jack-knife very quickly.
On dump trucks the center of balance is
very high, and if the truck gets very sideways in
a skid
it is likely to roll. My point is a truck driver
probably
wants to keep the braking force below the point of
having
the brakes lock. ( Anyone know if there are
antilock
brakes for 18 wheelers?)

Thanks
Roger Haar





Eugene Mosca wrote:
> 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