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Homework Help: Stupid Speed Question

  1. Jan 20, 2010 #1
    1. The problem statement, all variables and given/known data

    A car travels up the hill at a constant speed of 30 km/h, then returns down hill at the speed 66 km/h. Calculate the average speed round trip

    I can't believe I am stumped on such a kindergarten problem, I hate online homework and the garbage system they are designed on.


    2. Relevant equations



    3. The attempt at a solution

    [itex]\frac{66+30}{2} =48 km/h[/itex]

    Says I am wrong, which I prolly am, then states the average speed is the ratio of the total displacement over time, obviously, then says use to D to symbolize distance if need be...?
     
  2. jcsd
  3. Jan 20, 2010 #2

    berkeman

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    Staff: Mentor

    I think you just need to pick an arbitrary distance and calculate how long each leg of the trip takes, and use those numbers to figure out the average speed...
     
  4. Jan 20, 2010 #3
    Another hint: The way you average is as if the car was going the same amount of time at 30km/h and 66km/h.
     
  5. Jan 20, 2010 #4

    berkeman

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    Staff: Mentor

    Really? But it isn't taking the same amount of time of course, since it it going the same distance at the two speeds in this question. Still, I'll wait until the OP posts his work to see if your way works too.
     
  6. Jan 20, 2010 #5
    Hmm as if the car is going the same amount of time at both speeds....
    This question is just a tad confusing, pick an arbitrary distance to find the how long each trip takes

    something like this? Say I pick 25 km for the distance

    [itex]30\frac{km}{h} = \frac{25 km}{t}[/itex]
     
  7. Jan 20, 2010 #6

    berkeman

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    Yeah. Find t1 and t2 to get your total time, and you know your total distance.... And then check it again for some other arbitrary distance. Do you get the same average speed?
     
  8. Jan 20, 2010 #7
    Ok, lets see here for the first constant speed of 30 km/h, I am gonna pick an arbritary distance of 25 km for both speed constants

    [itex]Speed:30 km/h[/itex]

    [itex] Distance: 25 [/itex]

    [itex]Time: 0.83[/itex]

    Next speed constant

    [itex]Speed: 66 km/h[/itex]

    [itex]Distance: 25 [/itex]

    [itex]Time:0.3787[/itex]

    Average Speed
    [itex]\frac{25}{1.2087}=20.68 km/h[/itex]

    Picking another arbitrary distance say 60 km

    My average speed equates to 20.62 km/h

    Pretty close I'd say, and all I had to do is that? Wow, and the keyword in the problem was "return"?
     
  9. Jan 20, 2010 #8

    berkeman

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    Staff: Mentor

    Good job!
     
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