1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Kinematics interpretations

  1. Sep 21, 2011 #1
    Please interpret this problem as you will. My professor and I seem to have different interpretations of this problem.

    A runner hopes to complete the 10, 000 meter run in less than 30 minutes. After exactly 27.0 minutes, there are still 1100 meters to go. For how many seconds must then the runner accelerate at 0.20 m/s2 to achieve the desired time?

    3. The attempt at a solution
    My interpretation was that he can run the entire 10,000 meters in LESS than 30 minutes. So this is what I did:

    1. I found that starting from rest, he can run the first 8900 meters in 27 minutes.

    10000-1100 = 8900 m
    t = 27(60) = 1620 seconds

    Thus the average velocity is v = x/t = 8900/1620
    v=5.49382716 m/s

    2. I know that when this velocity is reached, he starts to accelerate at a = 0.2 m/s2. So I call it Vi.

    Starting from Vi = 5.49 m/s, he has 1100 meters left to run.
    Using the equation x = xo + Vit + 0.5at2 I get
    1100 = 5.49t + 0.5(0.2)t2

    Cleaned up you get
    0.1t2 + 5.49t - 1100 = 0
    Solve. T = 80.94928247

    Is my answer correct? My teacher's answer is different than mine. Did I misinterpret the problem somewhere? How is your interpretation? Thank you!
  2. jcsd
  3. Sep 21, 2011 #2


    User Avatar
    Homework Helper

    Re: Kinematics

    Your approach assumes the acceleration remains until the race is finished, so the runner's total time is ~28:20. I suspect the intent of the problem is to find out how long the runner needs to accelerate, then coast at the final velocity, to finish the race in exactly 30 minutes. I know the problem statement say's 'under', but how much under isn't stated, so I think the problem could have been worded better.
  4. Sep 21, 2011 #3
    Re: Kinematics

    I believe you are correct. Her email said that. I would never have interpreted the problem that way :( let's hope the quiz isn't like that! Thank you so much!
  5. Sep 21, 2011 #4


    User Avatar
    Homework Helper

    Re: Kinematics

    One thing you can do in a situation like this is state that the probem is not clear and why, then list the assummptions you choose to use (because the wording isn't clear) and solve the problem. You might get partial credit if you solve a different problem than what was intended but make a good logical case.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook