Velocity/acceleration/time to stop

  • Thread starter Thread starter lauriecherie
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Homework Help Overview

The problem involves calculating the time required for a car to decelerate from a speed of 132 km/h to below the speed limit of 90 km/h, given a deceleration rate of 4.9 m/s². The context is within kinematics, specifically focusing on velocity and acceleration.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss the need to convert speeds from km/h to m/s, with one participant questioning the calculations leading to an initial time estimate of 8.57 seconds. Another participant suggests that the correct time should be between 2 and 3 seconds.

Discussion Status

The discussion is ongoing, with participants providing feedback on calculations and emphasizing the importance of unit conversion. There is a shift in focus as one participant revises their approach based on the feedback received.

Contextual Notes

There is a noted assumption regarding the conversion of speed units, which appears to have impacted the initial calculations. The original poster acknowledges this oversight and is re-evaluating their solution.

lauriecherie
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Homework Statement



The brakes on your car are capable of slowing down your car at a rate of 4.9 m/s^2.
If you are going 132 km/h and suddenly see a state trooper, what is the minimum time in which you can get your car under the 90 km/h speed limit?

Homework Equations



v(t)= at + initial velocity

(final velocity^2) - (initial velocity^2) = 2*a*delta X (change in position)

The Attempt at a Solution



I came out with 8.57 seconds. Is there some converting I maybe missed?
 
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Hi Laurie, welcome to PF!
I don't know how you got 8.57. I got between 2 and 3s. Show your calc and we'll find your error.
Did you remember to convert the speeds to m/s ?
 
Delphi51 said:
Hi Laurie, welcome to PF!
I don't know how you got 8.57. I got between 2 and 3s. Show your calc and we'll find your error.
Did you remember to convert the speeds to m/s ?


No, I didn't convert. Whoops. So let me work this out again. Thanks for the advice.
 
2.380952388 seconds. :!) Thanks soooo much for your help!
 
Most welcome!
 

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