Does anyone know the formula for estimating speed by the length of the skid marks?

Question

Does anyone know the formula for estimating speed by the length of the skid marks?

Answer 1

There is probably, undoubtedly, a formula for determining the speed of a car from length of the skid marks, but it would have to take into account too many variables for any accuracy.

Different tires would have different coefficients of friction as would the road surface. This would change depending on the tires' inflation as well as rubber compound used.

Were the tires properly inflated?

Road surface is also a determining factor. Was one or all of the tires slightly wet? Or very wet? Was the road surface dirty or gravely?

Answer 2

S=SQRT30Df

where,

S = speed in miles per hour (MPH),

D = skid distance, in feet, and

f = drag factor.

Answer 3

Basically what you are trying to figure is based on the length of the skid marks, what friction force brought the vehicle to a stop in that distance, only in reverse.

You already know what the coefficient of friction is (dry tires on pavement from your list of tables).  You also know what distance it took to take the velocity and acceleration from ? to zero (the ? being what you are after).

So basically, your equation is going to take into account the force of the car (F=ma=mx?, since this has your unkown quantity in it), the distance that it took to stop, the friction and gravity force acting on the tires.

Since you know that the static friction force is higher than the kinetic force for sliding (once something starts sliding, the friction coefficient goes down), you want to know what the static force is and when your coefficient of friction reaches that, then you will have the force necessary to have stopped the car in that distance.

So:

F(static friction) = F(normal), since once the car is sliding, it doesn't exert any other force except the force normal to the surface (it's weight/mass), but when the friction stops it is when the sliding friction suddenly changes to the static friction force.

Then:

F(static) = u(static) x F(normal)

Through substitution using the change in kinetic energy and work, you will come to the equation:

v = sqrt (2ug)

where

v=velocity (speed)

u=static friction coefficient

g=gravity (weight of car)

See these sites for more info:

http://www.intuitor.com/student/Descript...

http://hyperphysics.phy-astr.gsu.edu/hba...