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Homework Help: Average velocity (should be easy )

  1. Jan 21, 2010 #1
    Average velocity (should be easy....)

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

    In a relay race, each contestant runs a distance 25.1m while carrying an egg balanced on a spoon, turns around, and comes back to the starting point. Edith runs the first distance 25.1m in a time = 19.1s. On the return trip she is more confident and takes only time = 14.1s.

    2. Relevant equations

    3. The attempt at a solution

    What is her average velocity for the entire round trip????

    so i found the avg vel of her 1st trip.... 25.1m / 19.1s = 1.31 m/s
    then i found the avg vel of her 2nd trip..... 25.1m / 14.1s = 1.78 m/s

    So then wouldnt the avg vel of the entire trip just be
    .... (1.31+1.78) / 2 = 1.55 m/s

    This is wrong though..
  2. jcsd
  3. Jan 21, 2010 #2
    Re: Average velocity (should be easy....)

    It's important to distinguish between average velocity and average speed.
    I think the question wants you to find the average speed. I'll explain why in a moment.
    Average speed is total distance travelled divided by total time taken. It's not calculated as the average of the two speeds over the two halves of the journey.
    To find the answer, you need to know the total distance run, and divide it by the total time taken.

    Average velocity is total displacement divided by total time and is a vector.
    The total displacement is zero because the runner returns to the start. The average velocity is therefore zero.
    Check what the question actually asks for.
  4. Jan 21, 2010 #3
    Re: Average velocity (should be easy....)

    It does say average velocity and the answer was 0. I wasnt thinking that displacement was zero i was thinking it was 50.2 (the total distance).

    Thank you for clearing that up.
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