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Kinematics Eq'n Problem

  1. Sep 17, 2013 #1
    1. The problem statement, all variables and given/known data
    A student riding a bicycle begins to go downhill and accelerates at a rate of 1.8m/s2. If the acceleration lasts for 2.4s, and the final speed of the bicycle is 10.2m/s, at what speed was he initially travelling?

    a = 1.8
    t = 2.4
    vf = 10.2

    vi = ?

    2. Relevant equations
    a = vf - vi / t


    3. The attempt at a solution
    I first isolated vi to solve for the problem and got : vi = vf-a/t and then plugged in the variables but it didn't work.
     
  2. jcsd
  3. Sep 17, 2013 #2

    SteamKing

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    a = (vf - vi) / t is the correct equation. Parentheses make a difference. Also, using correct algebra.

    However, this equation is good only over short time intervals.
     
  4. Sep 17, 2013 #3

    collinsmark

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    Be careful with your parenthesis, they make a difference.

    the correct equation for uniform acceleration is

    a = ( vf - vi )/t

    There's an algebra mistake in there somewhere. It shouldn't contain the term a divided by t. It's something else.

    [Edit: SteamKing beat me to the response.]
     
  5. Sep 17, 2013 #4
    Ok, then is this correct?:

    a = (vf - vi)/t
    at = vf - vi
    at + vi = vf
    vi = vf - at
     
  6. Sep 17, 2013 #5

    SteamKing

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    Looks Good
     
  7. Sep 17, 2013 #6
    Ohh, I was so confused since I thought I had to divide t from both sides since it was a*t...
    Anyways, thank you!!
     
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