I\'ve worked this problem 3 different times, with 3 different answers... A car i

Question

I've worked this problem 3 different times, with 3 different answers...

A car is travelling at 45 mph; the driver sees a stop sign 150 ft ahead and hits the brakes, and decelerates at 11.4 ft/s2. How fast is the car going as it passes the stop sign?

I've calculated that it takes 5.8 s to come to a complete stop, and the distance travelled is 191 ft. But the velocity as it passes the stop sign I'm getting 1.93 s, about 4 s, and over 8 s...I'm fairly certain the last one has to be wrong. Unless I miscalculated the time & distance it takes to stop of course. Help!

Explanation / Answer

intail velocity u = 45 mph =20 m/s        (1 mph = 1601m/3600s)

s = 150 ft = 46.15 m           ( 1m = 3.25 m)

a = -11.4 ft /s^2 = - 3.50 m/s^2

v =?

we know

v2 - u2 =2as ,

v2 = 76.40

solving we get v= 8.73 m/s

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