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More help needed

  1. Sep 23, 2005 #1
    this problem deals with freely falling bodies

    A woman is reported to have fallen 144 ft from the 17th floor of a building, landing on a metal ventillation box, which she crushed to a depth of 18.0 in. she suffered only minor injuries. Neglecting air resistance, calculate (a) the speed of the woman just before she collided with the ventillator, (b) her average acceleration while in contact with the box, and (c) the time it took to crush the box.

    wow im so lost. how do u use the 17th floor anf dent made in the ventillator??? can someone start all 3 parts for me?
  2. jcsd
  3. Sep 23, 2005 #2


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    a) For uniform acceleration:
    [tex]x = x_0 + v_0t + \frac{1}{2}at^2[/tex]
    and [tex]v = v_0 + at[/tex]
    Or you can use conservation of energy.
    b) How is average acceleration defined?
    c) see a)
  4. Sep 23, 2005 #3
    still lost...

    i plugged in: (18in)=(1728in)(<---converted to inches)+[tex]v_0[/tex]+(1/2)(9.8m/s^2)(t)

    what now?
  5. Sep 23, 2005 #4


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    You missed two t's.
    Calculate the speed just before she collides. That means x = 0.
    From the fisrt equation, solve for time. Hint: [itex]v_0[/itex] = 0. You know what a is, what is it?
  6. Sep 23, 2005 #5
    a=-9.8m/s correct?
  7. Sep 23, 2005 #6


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    That is correct.
  8. Sep 23, 2005 #7
    :cry: what do u mean i missed 2 ts?
  9. Sep 23, 2005 #8
    also, when you plug into the equation, both x and [tex]v_0[/tex]=0 correct?
  10. Sep 23, 2005 #9


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    Notice that you are given distances in inches and feet, if you want to use 9.8 m/s^2 as g, you need to convert all distances to meters.
  11. Sep 23, 2005 #10
    ok, i got a right. now how do u do b? i no that the equation is deltav/delta t
  12. Sep 23, 2005 #11


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    (18in)=(1728in)(<---converted to inches)+[itex]v_0[/itex]+(1/2)(9.8m/s^2)(t)
    Instead of [itex]v_0[/itex] you should have [itex]v_0t[/itex] and instead of (t) you should have [itex]t^2[/itex].

    b) Well actually, I suppose this is an easier approach:

    [tex]x = x_0 + v_0t + \frac{1}{2}at^2[/tex]
    [tex]v = v_0 + at[/tex]
    Can you see it?
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