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Average speed question

  1. Sep 12, 2007 #1
    1. The problem statement, all variables and given/known data
    A car is moving at a constant speed of 13 m/s
    when the driver presses down on the gas pedal
    and accelerates for 11 s with an acceleration
    of 1:4 m/s2.
    What is the average speed of the car during
    the period? Answer in units of m=s.

    3. The attempt at a solution

    using the formula distance = Vi t + 1/2 A t squared
    i solve for distance and then just simply divide that by the 11 seconds?
    Last edited: Sep 12, 2007
  2. jcsd
  3. Sep 12, 2007 #2
    Note sure what you mean by 1:4 m/s^2 -- But your solution looks right
  4. Sep 12, 2007 #3

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    No. Average speed is defined as [itex]\Delta d/\Delta t[/itex], where [itex]\Delta t[/itex] is the duration of the time span and [itex]\Delta d[/itex] is the distance traveled during this span. If you average the speeds at arbitrary time points over the time interval you will get a different (and incorrect) answer.

    Edited to add:
    As you divide the interval into ever small chunks, the average of the speeds does approach the average speed. It is far easier to calculate the average speed as defined.
    Last edited: Sep 12, 2007
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