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Simple Motion Questions [Grade 10 Level]

  1. Mar 9, 2009 #1
    1. The problem statement, all variables and given/known data
    Sylvester is training for a boxing match. He runs due north at an average speed of 16 km/h for the first 8 km and returns to the training camp at an average speed of 10 km/h. Calculate Sylvester's average speed for the run and his average velocity.



    2. Relevant equations
    distance = (speed1+speed2)/2 (time)
    distance = velocity x time


    3. The attempt at a solution
    I don't know how to go about this when they have not provided the time. Help?
     
  2. jcsd
  3. Mar 9, 2009 #2

    tiny-tim

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    Welcome to PF!

    Hi myusername! Welcome to PF! :smile:
    they're expecting you to work out the time :wink:

    how would you do that? :smile:
     
  4. Mar 9, 2009 #3
    Hi
    This question needs only s=d/t
    Split into 2 parts, journey north, and journey south.
    Journey north is for 8km at 16km/h.
    therefore t=d/s = 8/16 = 0.5 hours.

    Journey south at 10km/h, we know must again be 8km,
    therefore t = d/s = 8/10 = 0.8 hours

    To get average speed, use total distance travelled and total time:
    t(total) = 0.5 + 0.8 = 1.3 hours
    d(total) = 8 + 8 = 16 km
    therefore:
    s = d/t = 16/1.3 = 12.31 km/h

    The average velocity is zero, because velocity is a vector quantity (it has a value and direction). The overall displacement (vector quantity of distance) is zero, because he finishes where he began, therefore zero overall displacement gives zero average velocity.

    Thats one of your questions done, hope this helps.
     
  5. Mar 9, 2009 #4

    tiny-tim

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    Hi nick_d_g!

    Please don't give out full answers on this forum :smile:
     
  6. Mar 9, 2009 #5
    Thanks very much. (:
     
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