A vehicle is test skidded on a level asphalt surface (asphalt f-0.70) and then o

Question

A vehicle is test skidded on a level asphalt surface (asphalt f-0.70) and then on an adjacent gravel shoulder (gravel f-0.45) where it finally comes to a halt. The length of the skid mark on the asphal sur face was 130 feet and on the gravel shoulder 30 feet. What was the speed of the vehicle at the beginning of the skid? 2 The driver ofa car unexpectedly sees a disabled stopped truck in front of it. The driver cannot swerve around the truck since it completely blocks the road. The car collides with the truck after leaving 20-ft skid marks on the pavement. Based on the damages sustained, an engineer estimated that the speed at collision was 15 mph. Calculate the speed of the car at the onset of skidding. Use a pavement friction factor value of f-0.6 and a level grade.

Explanation / Answer

1. test skidfding vehicle

uk = f = 0.7

ukg ( gravel) = fg = 0.45

skid marl length, asphalt = da = 130 ft

skid mark length, gravel, dg = 30 ft

hence

from energy conservation

uk*m*g*d = 0.5mv^2

v = sqrt(2f*gd)

initial speed = v

then

from eonservation of energy

m*g(f*da + fg*dg) = 0.5mv^2

v = 82.03535 ft/s

2. skid mark, l = 20 ft

vc = 15 mph = 22 fps

f = 0.6

then

intial speed = v

f*g*l = 0.5(v^2 - vc^2)

v = 35.451375149 ft/s

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