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Average velocity from multiple displacement/velocities

  1. Nov 27, 2009 #1
    Heres the problem:
    "A car travels at a constant speed of 60kph for 30km, 40kph for another 30km and 50kph for the final 30km. What is the average speed of the car."

    I know that in this case I can just add the velocities and divide by 3 since the displacements are equal and get 50kph as the average speed but when I tried to do it the long way I didn't get the same answer. First I calculated the total time elapsed like this d1/v1 + d2/v2 + d3/v3 and got t = 1.85h. Then to get the average speed I divided the total displacement by time (90/1.85) and got 48.65kph.

    Theres a pattern there I've tried this with various problems and I always a little less than the right answer. Whats going on here?
  2. jcsd
  3. Nov 27, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Although it might seem right, it's not. You really need to find total displacement over time.

    Now if the time intervals were equal, then it would work. :wink:
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