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PLEASE HELP Physics HW Question

  1. Feb 24, 2006 #1
    The referee throws a jump ball vertically in the air. Besides the force
    of gravity, the drag force acts on the ball. The drag force is given by
    the product of the friction coefficient with the square of the velocity.
    The friction coefficient is 0.1. The NBA ball mass is 0.6 kilograms. The
    initial ball velocity is 10 meters/second. How much time does it take
    for the ball to slow down to 2 meters/second?

    So do you set
    Fdrag = Fgravity

    So far I have Fdrag = u x v^2
    where Vi=10 and Vf=2 a= 9.8

    I'm really bad at physics can some Please help me

  2. jcsd
  3. Feb 24, 2006 #2


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    This is more mathematics than physics. Set up the equation of motion (F=mdv/dt), solve it. Find the particular solution by plugging your inital conditions. Set v = 2. Solve for t. You'll get two answers; trash the larger t.

    The only physics involved is knowing why you trash the larger t.
  4. Feb 26, 2006 #3
    still don't understand
  5. Feb 26, 2006 #4


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    What part don't you understand? Can you post and show the steps you have taken till now?
  6. Feb 26, 2006 #5

    so i set t=0 and Vi=10 and solve for C then plug in Vf and solve for t?

    C=-1/10 but according to the second thread i'm suppose to get 2 t's.
    Last edited: Feb 26, 2006
  7. Feb 26, 2006 #6


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    First of all, the drag force acts in the opposite direction of velocity. So since the ball is thrown up, and it is natural to take the y axis to point upward, you'll need to take [itex]F_{drag} = -uv^2[/itex].

    Secondly, you're forgetting to account for the gravitational force in your equation of motion.
  8. Feb 26, 2006 #7
    doesn't m*dv/dt that's gravity



    is so how can you isolate the v's on oneside?
  9. Feb 26, 2006 #8
    first of all mg and the drag force is in the same direction so your Fnet=-mg-uv^2 = m dv/dt and then you need to get red of m by dividing both side with. Since you need to integrate the equation .
    you should write it as V`s on one side on the equation and t`s on the other side so that you can integrate it
    dV/-(9.8+uv^2/m)=dt and just integrate both sides ( v0 to V ) and (to=0 to t )
  10. Feb 26, 2006 #9
    thanks i got 0.52 secs..did anyone else who bothered to work out the problem in its entirety get that also
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