Question

A motorist is driving at 18.1 m/s when she sees that a traffic light 192 m...

A motorist is driving at 18.1 m/s when she sees that a traffic light 192 m ahead has just turned red. She knows that this light stays red for 14.6 s, and she wants to reach the light just as it turns green again. It takes her 0.9 s to step on the brakes and begin slowing. What is her speed as she reaches the light at the instant it turns green?

d= Vi(t)+ 1/2a (t2)

173.9=18.1(13.7) +1/2a (13.72) ---> 173.9= 247.97 +1/2a (187.69) --->

-74.07= 1/2a(187.69) ---> -0.3946= 1/2a(187.69) ---> a= -0.78928

Vf= 18.1+(-0.78928)(13.7) ---> Vf= 7.2869 m/s (wrong answer)

I have also tried these answers (7.29 m/s , 7.3m/s, 16mph, 9.93m/s)

I have tried solving this 2 different ways and it keeps saying the answer is wrong

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Homework Answers

Answer #1

Solution:

Given that

u = initial speed =18.1m/s

x = distance travelled = 192 m

t = total time = 14.6 s

The distance travelled by motorist in 0.9 s is

The distance left to travel

Time left

The motorist now apllies brake, hence

175.71=18.1 *13.7-0.5*a*13.72

The final speed

v=18.1-.769*13.7

Try this i hope it will help you.

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