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Homework Help: Avg Speed

  1. Oct 2, 2013 #1
    1. The problem statement, all variables and given/known data
    A car travels to home from school. For the first 100km it travels 100km/h and for the rest of trip travels 50km/hr and completes the trip in 1 hr and 42 min. Calculate the avg speed for the ENTIRE trip.

    2. Relevant equations
    Average Speed=∑i|d/Δttotal

    3. The attempt at a solution
    It's asking for the entire trip so we assume it comes back home.

    In this case is avg speed = 0 because the displacement was 0.
    Last edited: Oct 2, 2013
  2. jcsd
  3. Oct 2, 2013 #2
    It looks like an incomplete text. Is this the actual text or a "summary"?
  4. Oct 2, 2013 #3
    I'm just wondering if it is correct
  5. Oct 2, 2013 #4
    Aha, you modified the text.
    Now it makes sense. And you can calculate the average speed.
    The trip is exactly what they say: from school to home.
    Calculate the total distance traveled and divide by the total time.
    It may some room for interpretation when they say "completes the trip in 1 h and 42 min".
    He did the first 100 km (in 1h) and then it takes another 1h 42min to finish the trip?
    Or he completed the whole trip in 1 h and 42 min, which will leave 42 min for the portion traveled with 50 km/h.
  6. Oct 2, 2013 #5
    so what I said is wrong? We have to assume they came from home to school. The actual question bolds the words "entire trip"
  7. Oct 2, 2013 #6


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    No, this is not a valid assumption. No info is given for the home to school trip. And besides, average velocity and average speed are not the same when traveling round trip. By the ENTIRE trip it means both sections of the one way trip from school to home.
  8. Oct 2, 2013 #7
    so then the avg speed would be 75 km/h
  9. Oct 2, 2013 #8
    Average speed is not the average of the two speeds. You need to use the formula you wrote yourself. Total distance/total time.
  10. Oct 2, 2013 #9
    Forgive me guys, this was a test question I'm just going off memory. But a friend told me that it said the person drove from home.

    So then the entire trip would be to the school and back.

    If this is the case, then is the avg speed 0m/s?
  11. Oct 2, 2013 #10


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    If the person drove from home to school and back to home, the average velocity (total displacement divided by total time) would be 0. Average speed is total distance traveled divided by total time.
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