Answer
The average horizontal force exerted on the automobile during braking is $6000~N$.
Work Step by Step
The impulse exerted on the automobile is equal to the change in momentum. We can use the magnitude of the change in momentum to find the average force on the automobile:
$F~t = \Delta p$
$F~t = m~\Delta v$
$F = \frac{m~\Delta v}{t}$
$F = \frac{(1.0\times 10^3~kg)(30.0~m/s)}{5.0~s}$
$F = 6000~N$
The average horizontal force exerted on the automobile during braking is $6000~N$.
