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Homework Help: Finding distance that a pad is compressed after being dropped

  1. Dec 2, 2008 #1
    1. The problem statement, all variables and given/known data

    An egg is dropped from a third-floor win-
    dow and lands on a foam-rubber pad without
    The acceleration of gravity is 9.81 m/s2 .
    If a 56.3 g egg falls 11.9 m from rest and the
    7.29 cm thick foam pad stops it in 6.32 ms,
    by how much is the pad compressed? Assume
    constant upward acceleration as the egg com-
    presses the foam-rubber pad. (Assume that
    the potential energy that the egg gains while
    the pad is being compressed is negligible.)
    Answer in units of m.

    2. Relevant equations

    I'm not quite sure. I was thinking EP (mgh) = 1/2 kx2, but you aren't given the spring constant.

    3. The attempt at a solution

    I calculated that just before the egg hit the pad, its velocity was 15.28m/s. After this...I have no clue where to go.
  2. jcsd
  3. Dec 2, 2008 #2
    I think you probably have to do something with the impulse that the pad delivers to the egg:
    The egg hits the pad with some initial velocity and then is brought to a complete stop before most likely bouncing up a little bit.
    This is essentially an acceleration in the upward direction. Then you can use the equations:
    v(final)=v(int) + at
    to solve for your two unknowns.
    Hope this helps, sorry my equations are not that easy to read (d=delta)
  4. Dec 2, 2008 #3
    I think my brain is fried after doing physics for a few hours straight...but your first equation doesn't seem to make sense to me.


    Two equals signs? How would you solve for x?
  5. Dec 2, 2008 #4
    haha sorry the second equals was supposed to be a plus. my bad. Yea solving for x would have been interesting.
  6. Dec 5, 2008 #5
    Tried that, got an answer of .144854 m, but according to my online assignment page, that's wrong. Is there a problem in my math, or is that the wrong formula to use?
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