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Thread: Confusion between average speed and average acceleration

  1. #1 Confusion between average speed and average acceleration 
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    A motorist drives 150km from one city to another in 2.5h, but makes the
    return trip in only 2.0h. What are the average speeds for
    a. Each half of the round-trip and
    b. The total trip?

    To find average speed, you do total distance / total time . For this question, I found the individual speed as 17m/s and 21m/s. and so if I do v1 + v2 ) / 2 , i happened to get the answer for b. but the actual formula to use is total distance / time right.. i'm conufsed about that.

    Second, for average acceleration.
    t = 1 d= 10
    t= 2 d= 30

    to find the avg a for this, why can't you do (30-10) / (2-1) / (2-1 ) but d2/t2 - d1/t2 / t2 - t1 , or why is it not total velocity / total time just like what average speed or velocity is.

    Third, why is avg vel = total displacement / total time but not change in displacement / change in time?

    Please answer this !! thank you very much!


    Last edited by cmkc109; February 4th, 2013 at 01:41 AM.
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  3. #2  
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    Quote Originally Posted by cmkc109 View Post
    A motorist drives 150km from one city to another in 2.5h, but makes the
    return trip in only 2.0h. What are the average speeds for
    a. Each half of the round-trip and
    b. The total trip?

    To find average speed, you do total distance / total time . For this question, I found the individual speed as 17m/s and 21m/s. and so if I do v1 + v2 ) / 2 , i happened to get the answer for b. but the actual formula to use is total distance / time right.. i'm conufsed about that.
    You cannot get the average speed by averaging the two speeds for each leg of the journey. This is because the slower leg of the journey will take more time. Calculate it both ways, and you will see that you get different answers.


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  4. #3  
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    thx for the reply, what about the second and third part of the ques?
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    I don't understand the second part of the question. Is this supposed to be the same trip as the first part of the question or a different trip? What are the units of t and d? If all you know is the distance at two times, there is no way to determine an acceleration.

    In the third part of the question, who says it isn't change in displacement / change in time?
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  6. #5  
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    Third, why is avg vel = total displacement / total time but not change in displacement / change in time?
    I don't know what distinction you are making between total and change in.

    Remember that velocity is a vector quantity and at the end of a return trip your displacement is zero. (I used to like confusing British Rail ticket clerks by asking for a return ticket. When they asked "where to" I replied "Well here, obviously.")
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