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Questin about moving an object through liquid

  1. Jan 25, 2009 #1
    1. The problem statement, all variables and given/known data

    Calculate the force required to pull a copper ball of radius 2.50 cm upward through a fluid at the constant speed 9.00 cm/s. Take the drag force to be proportional to the speed, with proportionality constant 0.950 kg/s. Ignore the buoyant force.
    1 N

    2. Relevant equations

    R=(1/2)DPAV^2

    3. The attempt at a solution

    the above formula may not be correct but its something like that. i tried to use that formula to calculate the force but the problem doesn't give me all the information required to plug it in to the formula. is there anything that i am missing taht you guys can guide me through
     
  2. jcsd
  3. Jan 25, 2009 #2
    Draw a free body diagram of the ball. There are three forces acting on the ball since the bouyant force is ignored. Because it's moving at a constant speed, the net force has to be zero.
     
  4. Jan 25, 2009 #3
    so one would be the force that we are finding out, there is also a weight force and drag force pointing down? would the equation on the OP be required?
     
  5. Jan 26, 2009 #4
    There is a V^2 in your equation. Note, that for whatever reason, the drag used is set proportional to the velocity, rather than velocity squared. I would begin by finding drag as a function of velocity.
     
    Last edited: Jan 26, 2009
  6. Jan 26, 2009 #5
    The upward force is what you are solving for. You know the two downward forces, gravity and drag. Set the upward force equal to the sum of the two downward forces. The radius of the object has nothing to do with this problem because you were given the drag coefficient.
     
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