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Terminal Velocity Given Acceleration at Instantaneous Velocity

  1. Jun 15, 2013 #1
    1. The problem statement, all variables and given/known data
    This is a repost of a homework like question. The previous thread I did not understand.
    In the assumption that drag is proportional to velocity, and when v = 20 m/s, a = 7.35 m/s^2, find the terminal velocity.

    2. Relevant equations
    The thread stated that the equation most relevant would be ma = mg - kv.

    3. The attempt at a solution
    I substituted all know values to make m(7.35) = m(9.8) - k(20), and attempted to solve, but the answer is 40 m/s.
     
  2. jcsd
  3. Jun 15, 2013 #2

    verty

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    Homework Helper

    How far did you get? Did you get stuck anywhere? It is too soon to give a hint, not enough has been done.
     
  4. Jun 15, 2013 #3
    None of that information is needed. The initial question is that it is dropped from a large height. The only information given is the information proveded (v and a).
     
  5. Jun 15, 2013 #4
    Never mind. I solved it by not replacing ma with 0 but by dividing by m first and replacing v with v^2.

    In these types of problems, when is v or v^2 used?
     
  6. Jun 15, 2013 #5
    Hmm. In the initial problem, the mass of the steel ball mentioned in the initial problem is invalid. The mass is one if the answer is 40m/s with given info.
    https://www.physicsforums.com/showthread.php?t=659382

    Either way, the work is done.
     
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