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Physics Problem, cant find mistake, okz help

  1. Sep 21, 2009 #1
    Here is the Problem: For a movie scene, an 85.0 kg stunt double falls 12.0 m from a building onto a large inflated landing pad. After touching the landing pad surface, it takes her 0.477 s to come to a stop. What is the magnitude of the average net force on her as the landing pad stops her?

    IS THIS CORRECT, wheres the mistake?

    First, find the speed of the stunt woman as she hits the landing pad:

    v² = v₀² + 2gΔy
    = 0 + 2(-9.80m/s²)(-12.0m)
    v = 15.3m/s

    Now, the rate of acceleration required to bring her to a stop is:

    a = Δv / Δt
    = (v - v₀) / t
    = (0 - 15.3m/s) / 0.477s
    = -32.1m/s²

    So, the force that stops her is:

    F = ma
    = 85.0kg(-32.1m/s²)
    = -2.73kN (-2730N rounded)

    You could also use the impulse momentum theorem:

    FΔt = Δp
    F = Δp / Δt
    = [mv(f) - mv(i)] / t
    = m[v(f) - v(i)] / t
    = 85.0kg(0 - 15.3m/s) / 0.477s
    = -2730N (rounded)

    Please give a through answer
     
  2. jcsd
  3. Sep 21, 2009 #2

    berkeman

    User Avatar

    Staff: Mentor

    Your first method looks okay to me (didn't check the 2nd). I would call the force positive, though, since it's pointing up to decelerate her...
     
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