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Physics Question About Drag Force

  1. Sep 10, 2015 #1
    The problem statement, all variables and given/known data
    Consider two blocks connected by a light, inextensible string over a light pulley
    as follows:

    dpzu2x.jpg

    The slope is frictionless and the masses are released from rest. The mass m1 is
    3 kg and the mass m2 is 4 kg where m2 is submerged completely (from the
    beginning) in a liquid of density 1000 kg/m3. Take the density of m2 to be
    8000kg/m3 and the drag force (in Newtons) to be FD = 7v, find the terminal
    velocity of m2. (Hint: Don’t forget to include upthrust.)

    The attempt at a solution

    m1*g*sin50 + Fupthrust - fdrag - m2*g = (m1+m2)*a, when it reaches terminal velocity, acceleration becomes 0

    22.5 + 4.9 - 39.2 -Fdrag = 0
    Fdrag = -11.8
    7v = -11.8
    v = -1.68 m/s

    The solution from my prof is 1.68m/s, but I can't figure out why I got minus in the equation.
     
  2. jcsd
  3. Sep 10, 2015 #2
    I suggest you take a closer look at your values within your calculation.
     
  4. Sep 10, 2015 #3
    You need to think what the general direction you use in this question..
     
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