Re: [Phys-l] stopping distance

[Brian Whatcott <betwys1@sbcglobal.net>]

At 07:50 AM 1/5/2007, A Lapinski, you wrote:
> Using the Work-Energy Theorem, you can determine how the stopping distance
> is related to the car's initial velocity:
>
> W = KE
> fd = 0.5mv2
> mmgd = 0.5mv2
> mgd = 0.5v2
>
> If we assume the brakes "lock" the wheels to create a skid mark, then the
> above result shows that the stopping distance does NOT depend on the car's
> mass.
>
> However, students tell me and Driver Manuals often state that the weight
> of the vehicle IS a factor (heavier = longer stopping distance). In the
> above calculation, I am dealing with an idealized case, but in "real-life"
> anti-lock brakes change the actual stopping distance depending on the
> weight of the vehicle. I've also heard that large trucks have a unique
> braking system. Can anyone elaborate on these ideas?


It helps me to suppose that some kinetic energy is dispersed
as work in shearing the tires' footprints.
A heavy vehicle's tire footprint area does not scale directly with
its mass. Heavy vehicles use inflation pressures as high as 80 psi.
The braking friction surfaces are designed not to lock the
wheels excessively or easily.
   Hence, I assert that a heavy vehicle can dissipate a smaller
 proportion   of the vehicle's kinetic energy at the tire/road
 contact patch, and less  kinetic energy proportionately at
 the brake drum or disc.



Brian Whatcott    Altus OK    Eureka!