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How to calculate average velocity given distance and time?

  1. May 29, 2016 #1
    1. The problem statement, all variables and given/known data
    A motorist drives north for 35.9min at 73.3km/hr and then stops for 15.4min. He then continues north, traveling 114.5km in 1.93hr. What is his average velocity?

    2. Relevant equations
    avg velocity = xf - xi / time

    3. The attempt at a solution
    35.9 min * (1 hr / 60 min) = 0.5983 hr
    0.5983 hr * (73.3 km / hr) = 43 .858 km

    total time = 0.5983 hr + 1.93 hr + 0.257 hr = 2.785 hr

    avg velocity = xf - xi / time

    = 114.5 km - 43.858 km / 2.785 hr

    = 25.37 km /hr N

    which is actually incorrect because the answer is meant to be 56.9 km /h N.
    What am i doing wrong ?
     
  2. jcsd
  3. May 30, 2016 #2

    collinsmark

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    What is meant by "xf - xi" here, in general? And as applied to this problem?
     
  4. May 30, 2016 #3
    the displacement and in this problem, the final distance and initial distance
     
  5. May 30, 2016 #4
    That is where you have gone wrong I assume [itex]x_f[/itex] means final position and [itex]x_i[/itex] means initial position. Here 114.5 Km and 43.858 is already the displacement for the given time intervals. You can find the total displacement by adding the displacements.
     
  6. May 30, 2016 #5
    Wow thanks so much. If this question asked for average speed, would it be the same?
    because i know avd speed = total distance / total time
     
  7. May 30, 2016 #6
    Yes.
     
  8. May 30, 2016 #7

    haruspex

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    If all of the displacements are in the same direction, as they are here, average speed is the magnitude of the average velocity.
    Since the question asks for average velocity, you should include the direction in the answer.
     
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