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Homework Help: Parachutist problem!Need help where to begin!

  1. Apr 24, 2009 #1
    Here is the problem.

    The free-fall velocity of a parachutist can be simulated as

    du/dt=g-Cd/m u^2

    where u = velocity (m/s) t= time (s)
    g = acceleration due to gravity (9.81 m/s2).
    Cd = drag coefficient (kg/m),
    and m = mass (kg).
    For a 80-kg parachutist, solve this equation numerically from t = 0 to 30 s given that u(0) = 0. During free fall, Cd = 0.25 kg/m. However, at t = 10 s the chute opens, whereupon Cd = 5 kg/m.

    Please help! from were I should start solving this problem!
  2. jcsd
  3. Apr 24, 2009 #2


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    Homework Helper

    Two comments:

    1. In your class have you discussed numerical techniques to solve differential equations?

    2. Hint: think of it as two separate problems, one from t=0 to 10s and the second from t=10s to 30s. Care must be taken as to the initial conditions of each problem.
  4. Apr 25, 2009 #3

    Thank you for your reply,

    Here is my situation,

    I am at 2nd year of mechanical engineering. Due to economical and family problems I couldn’t attend classes for this year, so I took numerical analysis lesson as independent study. I am quite familiar with differential equations but not with numerical methods. My teacher told me that he needs from me not to solve differential equations by hand, but use of Matlab or Octave to solve "typical engineering problems». Like the problem above which I am very confused with it, I must solve it using Matlab, either way I spend more than 3 days of continuous reading trying to solve it also by hand with no luck.

    Thank you for your support.
  5. Apr 25, 2009 #4


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Perhaps the following example has enough info that you can see how to enter your problem into MATLAB:


    (Found by googling matlab differential equation solving, maybe you can find other useful info there)
    Last edited by a moderator: Apr 24, 2017
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