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: Long problem sum using newton second law

  1. Jul 24, 2012 #1
    1. The problem statement, all variables and given/known data

    Question is here http://postimage.org/image/som50onyv/ [Broken]

    2. Relevant equations

    3. The attempt at a solution

    for a) Power = force * speed

    for b) Force = Mass * Acceleration
    F = m * dv/dt

    for c) for maximum velocity, there must be zero drag

    i am stuck at D and E, so can anyone help me?
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jul 24, 2012 #2
    so what's the differential equation you get? Remember to include the drag term.

    If there is no drag, then there is no maximum velocity either (the car will just accelerate forever). You should get the answer to this by solving the differential equation you got from b).
  4. Jul 24, 2012 #3
    For part d you have 80000 watts of power available. Thus the car will accelerate until such time that the power consumed by resistance equals the 80000 watts. So what you have to do now is determine the power consumption due to resistance. From that you can determine the value of k.

    Hint: Units show you what to equate.
  5. Jul 26, 2012 #4
    for d) i got that k=50

    but am stuck for e) again.
    m dv/dt = (80000-50v²)/v

    thus m v (dv/dx) = (80000-50v²)/v

    and thus 1200v (dv/dx) = (80000-50v²)/v

    now do i integrate this to find the answer of part e? i'm kinda lost
  6. Jul 26, 2012 #5
    Just move all v's to one side and x's to the other side, then integrate. The integral is not completely trivial but it's still doable.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook