What are the forces involved in a brake test on dry concrete?

[Mike12345]

So I am stuck on another question I've been looking at

Ina brake test on dry concrete a Porsche Boxter traveling with an initial speed of 33.2 m/s can stop w/o skidding after moving 62.8 m . The mass of the Boxter including driver is 1340kg.

Determine the magnitude of the average accelertaion of the car during the non skidding braking

a = 87.76 m/s^2 <- i got this which i believe is correct

b) Cacluate the magnitude of the average stopping friction force

Ff ? Stuck here

c) Assume the test is now done with skidding on dry concrete. Calculate the stopping distance during the skid

Fk = 10,505,6 N
A = Fk/m = 7.84 m/s^2

Im stuck here too =S

 

[Delphi51]

a = 87.76 m/s^2 <- i got this which i believe is correct

This is way too large (driver will not survive). Better run it through again!

average stopping friction force

Friction force is the ONLY force acting on the car (horizontally). So use F=ma to find the total force on the car and you've got it.

skidding on dry concrete

The only difference here is that the car is slipping so you use the coefficient of kinetic friction instead of static friction. You must have a table of these coefficients so you can look it up. Then use the coefficient in the friction formula to calculate the force.