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Physics speed mph question

  1. Dec 12, 2004 #1
    I thought the answer was 90mph, but my friend says it's wrong. I just can't figure out any other answer. :cry:

    could someone please help me? :shy:
    You drive to the store which is a 2 mile trip. In the first mile you average 30 mph, how fast must you drive the second mile in order to average 60mph for the entire 2 mile trip?

    Jennifer :blushing:
  2. jcsd
  3. Dec 12, 2004 #2


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    I would have thought it would have been an average 90mph in the 2nd mile as well :confused:
  4. Dec 12, 2004 #3
    Nobody? :frown:
  5. Dec 12, 2004 #4
    If your average speed for the trip were 60 mph, it would take you 2 minutes to complete the entire trip. Thus you have to drive fast enough on the second leg to complete the trip in 2 minutes.

    However if you only drive at 30 mph for the first mile then it you will already have taken 2 minutes; no matter how fast you drive for the second mile, you'll take longer than 2 minutes so you can't actually average 60 mph for the whole trip.

    Note that this is because "average speed = distance travelled / travel time" and not the average of all the speeds over each segment.
  6. Dec 12, 2004 #5
    thnx! That makes sense! :)
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