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Velocity and speed

  1. Aug 28, 2008 #1
    1. The problem statement, all variables and given/known data
    A person walks first at a constant speed of 5 m/s along a straight line from point A to
    point B and then back along the line from B to A at a constant speed of 3 m/s. a. What is
    her average speed over the entire trip? b. What is her average velocity over the entire
    trip?


    2. Relevant equations



    3. The attempt at a solution
    I am confused with the total distance between A and B. As far as I know for the average
    speed I need to find the total distance from A to B and B to A. However how do I find
    that? I also need the time as well.
     
  2. jcsd
  3. Aug 28, 2008 #2

    LowlyPion

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    You can always solve it by defining some distance variable and calling it D.

    Note: They are asking you for Avg Speed and Avg Velocity. Do you know what the difference is between them?
     
  4. Aug 28, 2008 #3
    average speed is the total distance travelled divided by delta t and average velocity is delta x divided by delta t
     
  5. Aug 28, 2008 #4

    LowlyPion

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    What are the equations then for figuring the amount of time for the first leg and the second.

    I know you don't know the distance D between A and B, but put it in the equation anyway.
     
  6. Aug 28, 2008 #5
    that's my confusion, of finding out the time. I think the equation is s = v/t, so t is equal to v/s?
     
  7. Aug 28, 2008 #6

    average speed ... i came up with 4m/s... velocity... ?


    VE

    Edit: i'm sure this is wrong... certainly missing some subtelties...
     
  8. Aug 28, 2008 #7

    LowlyPion

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    That's the wrong equation.

    V = s/t which means t = s/V

    With S set to D and substituting the values of the velocities given then what is the total time? (time of the first leg plus time of the second)
     
  9. Aug 28, 2008 #8
    ok... 3.75 m/s... velocity ?...


    VE
     
  10. Aug 28, 2008 #9
    where did you get 3.75 from? and I know the average speed is 0 here as the total distance traveled is 0, as it went from A to B and B to A again, therefore they went back to the same place again where it started.
     
  11. Aug 28, 2008 #10

    LowlyPion

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    4 m/s is wrong.

    Actually not so subtle. The math lays it all bare. Follow where it leads.
     
  12. Aug 28, 2008 #11

    LowlyPion

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    No you have confused speed with velocity.

    Velocity is a vector. Magnitude and displacement.

    Speed is a scalar magnitude.
     
  13. Aug 29, 2008 #12

    LowlyPion

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    Do the math and you will see how to find the average speed.
     
  14. Aug 29, 2008 #13
    so you mean that average speed is not 0?
     
  15. Aug 29, 2008 #14
    first I need to find delta x and delta t, how do I find delta t? even I don't know the time
     
  16. Aug 29, 2008 #15

    LowlyPion

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    Average speed is not 0.

    Average velocity is 0. Velocity implies both direction of displacement as well as magnitude for the time taken.

    The first paragraph of this article might be instructive to you:
    http://en.wikipedia.org/wiki/Velocity
     
  17. Aug 29, 2008 #16
    Oh yea, sorry, I got that reversed.. now for the average speed. I need the total distance travelled, which is 2D and the delta t, which is??
     
  18. Aug 29, 2008 #17

    LowlyPion

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    V = x/t

    This means

    t = x/V

    If x=D how long to do the first leg?
     
  19. Aug 29, 2008 #18
    never mind, I got 3.75. Thanks all
     
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