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Homework Help: Problem using newtons second law

  1. Sep 19, 2011 #1
    Once her chute opens, a sky-diver of mass m is acted upon by a downward force Fg due to gravity, and an upward force Fr due to air resistance. If Fg = mg, where g is gravitational acceleration, and Fr is proportional to velocity v, use Newton’s Second Law of Motion to write acceleration a as a function of velocity v.

    I don't know if i'm on the right track
    Fnet = ma
    fnet = Fg+ Fr
    Fg = mg

    Mg + Fr = ma
  2. jcsd
  3. Sep 19, 2011 #2
    yes on right track except the signs. Now since [itex]F_r[/itex] is proportional to velocity v, we can write

    [tex]F_r=kv[/tex] where k is constant of proportionality.

    [tex]\Rightarrow mg-kv=ma[/tex]

    the negative sign is there since the air drag opposes the downward motion,

    [tex]\therefore a = g-\frac{kv}{m}[/tex]
  4. Sep 19, 2011 #3
    That makes a lot more sense, thank you.
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