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Homework Help: Bad Question - Average Acceleration

  1. Feb 1, 2014 #1


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    Gold Member

    1. The problem statement, all variables and given/known data

    A car starts from rest. With constant acceleration, the car increases its speed from 10m/s to 50m/s over a distance of 60m. How long does this take?

    Purported answer: 4.0s

    2. Relevant equations

    Constant acceleration, so the following equations apply:

    Vf^2 = Vi^2 + 2ad
    Vf = Vi + at

    3. The attempt at a solution

    Vf^2 = Vi^2 + 2ad

    2500 = 100 + 2a(60)
    2400 = 120a
    240/12 = a
    a = 20 m/s^2.

    Vf = Vi + at

    50 = 10 + 20(t)
    40/20 = 2s.

    Why does the key say 4s?

    I'm guessing it's because the key is wrong!
  2. jcsd
  3. Feb 1, 2014 #2


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    Science Advisor
    Homework Helper

    Yes, I think the key is wrong.
  4. Feb 1, 2014 #3

    D H

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    Staff Emeritus
    Science Advisor

    I concur with Dick. The key is wrong. Why? They might have changed the question from some previous edition and not changed the key. Or they might have solved a problem other than the one printed. Or they might have just made a mistake. It happens.
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