A man is driving at the speed $40 m p h$ when he see an obstacle at distance $300 f t$ ahead of his position. The driver applies the brakes and decelerates at $10 f t / s^{2}$ How long does it take him to stop the vehicle? (in s)

7

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6

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5

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Solution

The correct option is A $6$
Initial speed of the vehicle $u = 40 m p h$
We know that $1 m i l e = 5280 f t$ and $1 h = 3600 s$
$∴$ $u = 40 \times \frac{5280}{3600} = 58.7 f t / s$
Final velocity $v = 0$
Acceleration of the vehicle $a = - 10 f t / s^{2}$
Using $v = u + a t$
We get $0 = 58.7 - 10 t$
$⟹ t = 5.87 s \approx 6 s$

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