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Homework Help: Mass in the Air

  1. Oct 7, 2007 #1
    1. The problem statement, all variables and given/known data
    A stone with mass m = 0.9 kg is thrown vertically upward into the air with an initial kinetic energy of 270 J. The drag force acting on the stone throughout its flight is constant, independent of the velocity of the stone, and has a magnitude of 0.9 N. What is the maximum height reached by the stone?

    2. Relevant equations

    KE =1/2mv^2 and PE=mgh

    3. The attempt at a solution

    I am not sure how I should be going about this...I have KE and the mass so I figured I could plug those numbers into the KE-1/2mv^2 equation and solve for velocity, but then I am not sure how to get the height. could you please help me out? or guide me to the right equation?
  2. jcsd
  3. Oct 7, 2007 #2


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    That will get you the initial velocity. Then you either have to use energy methods; or else solve for the acceleration using newton 2, then use the kinematic motion equation to solve for the height. Are you familiar with either method?
  4. Oct 7, 2007 #3


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    The work done by the drag force is force*distance since it's constant. Add this to your other energy equations.
  5. Oct 7, 2007 #4
    Law of conservation of energy:
    Initial kinetic energy has changed to work done to overcome the drag force AND potential energy.
    Considering when the stone reached the maximum height:
    270 = Work done + Potential energy
  6. Oct 21, 2007 #5
    Use Conservation of Energy... Just Like he said : (.5mv^2 = mgh + F*h) the max height is in the variable h (height) when v (velocity) = 0
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