Stopping distance/time and constant deceleration

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Homework Help Overview

The problem involves a scenario where a driver must stop a car before reaching an intersection after a traffic light turns red. The context includes initial speed, reaction time, and the need to calculate the required acceleration to stop the vehicle at a specific distance.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the calculations related to stopping distance and acceleration, with some questioning the interpretation of reaction time and its impact on the stopping distance. Others express difficulty in determining acceleration and total stopping time.

Discussion Status

The discussion is active, with participants providing calculations and seeking clarification on the steps involved. Some guidance has been offered regarding the separation of reaction time from braking time, but there is no explicit consensus on the correct approach or calculations yet.

Contextual Notes

There is mention of a time constraint for the original poster, indicating urgency in resolving the problem. Additionally, some participants express confusion over the calculations leading to negative acceleration values.

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Homework Statement


You are driving to the grocery store at 18 m/s. You are 130 m from an intersection when the traffic light turns red. Assume that your reaction time is 0.50 s and that your car brakes with constant acceleration. What acceleration (magnitude) will bring you to rest right at the intersection? How long (total) does it take you to stop?



Homework Equations





The Attempt at a Solution

I got how far from intersection til brakes are applied... = 119.775 m
 
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How did you get the answer? Show your calculations.
 


vi=18 m/s t = 0.5 secs a= 9.8 m/s/s

0= 18 + a(.5)

= 10.225

130-10.225= 119.775m
 


0.5 s is the reaction time, not the total time.
During that time car moves 18*0.5 = 9 m. Then the brakes are applied.
Now the initial velocity is 18 m/s, distance is (130 - 9)m and final velocity is zero. Find acceleration and then time.
 


I am having a hard time finding acceleration and time...and this is due in ten mins...I keep getting -36m/s/s for acceleration...
 


Show your calculations. We have no idea how you are getting your answers if you don't show your work!
 

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