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Homework Help: Finding acceleration of a car when it is braking

  1. Apr 26, 2016 #1
    1. The problem statement, all variables and given/known data
    A 1200kg car moving at 20 metres per second stops in a distance of 50m when the brakes are applied.
    a) What is the acceleration of the car when it is braking?
    b) Find the force applied by the car's braking system.
    2. Relevant equations
    For a) I used the acceleration formula: a=(velocity - initial velocity)/t
    Time formula from rearranging: t=s/v

    3. The attempt at a solution

    velocity=50/2.5=20 metres per second

    a=(20-0)/2.5 = 8ms(^-2)

    I do not know how to find the acceleration when BRAKING, and the answer claims that it is 4ms(^-2.)

    As for b, I believe that I am unable to attempt it seeing as I am unable to find the acceleration.

    Please explain how to do it! I have read through the chapter of my textbook, and am unable to understand. thank you very much!

    Last edited: Apr 26, 2016
  2. jcsd
  3. Apr 26, 2016 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Braking to a stop is just the reverse of accelerating to speed from a standing start.

    Your equation t = s/v works only for constant speed. I have no idea what "2n5s" means.

    There is a SUVAT equation which relates acceleration, distance, and initial and final velocities which would be rather useful here.
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