science/math
posted by christina .
A car traveling at the speed of 30.0 m/sec encounters an emergency and comes to a complete stop. how much time will it take for the car to stop if its rate of deceleration is 4.0 m/sec

science/math 
Reiny
Using Calculus
a = 4
v = 4t + c
when t=0 , v = 30
30 = 0 + c
v = 4t + 30
s = 2t^2 + 30t + k
when t = 0 , s = 0, so k = 0
s = 2t^2 + 30t
when the car stops, v = 0
4t = 30
t = 30/4 = 7.5 seconds
when t = 7.5
s = 2(7.5)^2 + 30(7.5) = 112.5 m
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