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Homework Help: Resistive force

  1. Feb 4, 2004 #1
    I need to calculate the force required to pull a copper ball radius 2.00cm upward through a fluid at the constant speed 9.00cm/s. The drag forse is to be proportional to the speed, with proportionality constant .950kg/s. Ignore any boyant force.

    What I did so far was figure out the resistive force using the equation: R=-bv, where b= .950kg/s and v= .09m/s
    So that gives me -.0855Newtons.

    My equation set up so far is:
    SumYForces = mg+ Resitive force= Pull Force
    and then from there I have no clue, the answer in the back of the book is 3.01 N, but I just can't seem to get it. Thanks -anne
  2. jcsd
  3. Feb 4, 2004 #2
    How about the weight of the ball?

    PS: the unit of that proportionality constant is not kg/s. It must be N-s/m so that when you multiply it by m/s you end up with N.
    Last edited: Feb 4, 2004
  4. Feb 4, 2004 #3
    the weird thing is they give no mass, and thats what they give as the proportional constant.
  5. Feb 4, 2004 #4
    You are given the radius of the ball. From that, with a little effort, you can find the mass (and then the weight).

    As to the unit of the constant, you're right. Sorry, I guess I was too tired last night. (1kg/s)*(1m/s)=1(kg-m/s^2)=1N
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