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Calculate velocity at a certain time in an acceleration.

  1. Mar 6, 2015 #1
    A car accelerates from 36 km/h to 80 km/h o range of 500m.
    What's the speed at 250m?

    Answer with two value figures and the unit "km/h".


    Am I using the wrong formulas or something?
    What am I missing?

    36 km/h = 10 m/s
    80 km/h = 22,22 m/s

    avg. speed = 0,5(22,22 m/s + 10 m/s) = 16,11 m/s
    time = 500 m / 16,11 m/s = 31,03 s
    acceleration = (22,22 m/s - 10 m/s) / 31.03 = 0,39 m/s


    So I'm thinking like this (but it's wrong for some reason):

    10 m/s + 0.5 * 0,39 m/s * (31,03 s / 2) = 13,05 m/s

    13,05 * 3600 / 1000 = 46.98 km/h

    Answer: 47 km/h

    Any thoughts?
     
  2. jcsd
  3. Mar 6, 2015 #2

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Two thoughts:

    1) What makes you think the car is at the halfway point at half the time?

    2) What you have is a lot of numbers and it's very difficult to see what you're doing or to spot a mistake. It should be a lot easier if you work with ##suvat## symbols.
     
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