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Average Speed With Ambiguous Distance/Time

  1. Apr 6, 2016 #1
    1. The problem statement, all variables and given/known data
    A locomotive travels on a straight track at a constant speed of 40 mi/h, then reverses direction and returns to its starting point, traveling at a constant speed of 60 mi/h. What is the average speed for the round-trip?

    2. Relevant equations

    avgS = distance/time
    t = d/40

    3. The attempt at a solution

    avgS = 2d/(t+40/60(t))
    = 2d/(t+(2/3)t)
    = 2(t/40)/(t+(2/3)t)
    = (t/20)/(t+(2/3)t)

    I don't really know what to do after this point. The solution manual has the steps for the solution as:

    avgS = 2d/(t+(2/3)t)
    = 80t/(t+(2/3)t) = 48 mi/h

    I have no idea how they got to the last step. Any help is greatly appreciated.
  2. jcsd
  3. Apr 6, 2016 #2
    In case you don't know how they got the first step, they took the total distance over the total time: it's t+2/3t because he's travelling 1.5 times as fast on the way back. as t represents the amount of time it takes the train to get to the turning point, we can use the formula d = 40t (because distance = speed*time) and substitute into the formula given in the answers: 2d = 80t.
  4. Apr 6, 2016 #3
    Ah, thanks. How did they get the final answer of 48 mi/h?
  5. Apr 6, 2016 #4
  6. Apr 6, 2016 #5
    Thank you!
  7. Apr 6, 2016 #6

    Ray Vickson

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    Homework Helper

    I much prefer to do it from first principles, in detail and without skipping steps; here is how:

    If distance out = distance in = d (in miles), time out is T_o = d/40 (in hours), and time in is T_i = d/60. Total time = T_o+T_i = d(1/40 + 1/60) = (100/2400) d. Round-trip distance is 2d, so average speed is S_avg = 2d/(total time) = 2400/100 = 24 (in miles/hr).
  8. Apr 6, 2016 #7


    Staff: Mentor

    You have an error here, Ray, from a lost or misplaced factor of 2. The average speed for the entire trip is 48 mi/hr.
  9. Apr 6, 2016 #8

    Ray Vickson

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    Science Advisor
    Homework Helper

    Thanks. That was an obvious typo that I did not see before pressing the enter key.
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