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Motion Problem, Find Average Speed

  1. Sep 18, 2007 #1
    1. The problem statement, all variables and given/known data
    Julie drives 100mi to Grandmother's house. On the way to Granmothers, Julie drives half the distance at 40mph and half the distance at 60mph. On the returntrip, she drives half the time at 40mph and half at 60mph.

    What is her average speed on the return trip?

    2. Relevant equations

    v = v[0] + a dt
    s[f] = s[0] + v[0] dt + 1/2 a dt^2

    3. The attempt at a solution
    I need to find the time first I think and then do .5*40t + .5*60t but I'm not sure how to find t
     
  2. jcsd
  3. Sep 18, 2007 #2

    mgb_phys

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    Science Advisor
    Homework Helper

    You know speed, time and distance - nobody mentioned acceleration so you only need s = vt.
    Draw a sketch of the various distances, speeds and times - it will help you understand what you are trying to find.
    As a general tip - don't hit the equations until you know what you are trying to find!
     
  4. Sep 18, 2007 #3
    s1 + s2 = 100

    s1 = 1/2 * 40 t
    s2 = 1/2 * 60 t

    s1 = 100-s2




    100 - s2 = 1/2 *40 t
    s2 = 1/2 * 60 t


    s2 = - 1/2 *40 t + 100
    s2 = 1/2 * 60 t


    -20t + 100 = 30t

    10t = 100
    t=10


    (5*40 + 5*60 ) /10
    (200+300)/10 = 50 mph

    So basically like this?
     
  5. Sep 18, 2007 #4

    learningphysics

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    Homework Helper

    Was confused by your work... can you describe your steps. You need to find the total time taken. Average speed = total distance/total time = 200/total time.
     
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