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How do I solve these problems (kinematics)

  1. Sep 18, 2015 #1
    The problem statement, all variables and given/known data
    She drives for 40.0 min at 80.0 km/h, 10.0 min at 75.0 km/h, and 30.0 min at 45.0 km/h

    Determine the average speed for the trip



    The attempt at a solution
    I got 66.25khm by ((4*80)+(75)+(3*45))/8 which looks like its in the ballpark range but when I enter the answer for my online homework its incorrect.

    So whats the proper way im suppose to solve this?
     
  2. jcsd
  3. Sep 18, 2015 #2

    SteamKing

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    What fraction of an hour is 10 minutes?
     
  4. Sep 18, 2015 #3
    I appreciate the answer the question but I would enjoy more on understanding how to do them myself

    Other questions on my online homework I don't get
    Two boats start together and race across a 70-km-wide lake and back. Boat A goes across at 70 km/h and returns at 70 km/h. Boat B goes across at 35 km/h, and its crew, realizing how far behind it is getting, returns at 105 km/h. Turnaround times are negligible, and the boat that completes the round trip first wins.

    Boat A wins but by how much? What is the average velocity of the winning boat?
    since the boat starts moving at 70km/h and returns at 70km/h how come its says im wrongs when I enter 70?
     
  5. Sep 18, 2015 #4
    1/6th
     
  6. Sep 18, 2015 #5

    SteamKing

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    Why did you divide your calculation in Post #1 by 8 then?
     
  7. Sep 18, 2015 #6

    because they traveled for 80 minutes

    I don/t know the proper equation to punch these numbers into.
     
  8. Sep 18, 2015 #7

    SteamKing

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    The basic equation to use is distance = rate × time

    The only thing I can see that would be wrong with your original answer of 66.25 kph is that this figure has more significant figures than the original data. If you expressed it to 3 significant figures, that might be the difference between a correct result and an erroneous one.
     
  9. Sep 18, 2015 #8
    but my answer is correct right?
     
  10. Sep 18, 2015 #9

    SteamKing

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    Yes, except for the sig figs.
     
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