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A 1200 kg car going 30 m/s skids to a rest.

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3 Answers

Because the car completely stops you can apply the standard formula connecting the acceleration, distance traveled, initial and final speed. Let "a" be the acceleration, vi = 30m/s (initial speed), vf = 0 -final speed, F -force of friction,  "m" - mass. If the distnace traveled is "d", then we have
 
                                  2ad = vf2 - vi2 = -900 m2/s2
The force of friction is acting in opposite direction. Thus,
 
                       aF/m = - 6000 N/1200 kg = - 5m/s2
 
We obtain for d (after canceletaions)
 
                             d = 900m/2x5 = 90 m