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An odd one regarding impact force

  1. Dec 11, 2013 #1
    This is going to sound odd but I am hoping to use physics to defend an allegation made by my ex-wife. She alleges that I threw a knife into my kitchen sink and it bounced up and stuck into the ceiling (in reality I was overseas when the knife was broken by her brother - I'm just hoping to use as many angles of defense as possible).

    I think I have a basic understanding of the parameters I need to use but I am unsure of how to calculate some of those parameters. I guess it's about working backwards to calculate probability (I am hoping to show that the allegation is implausible).

    I know I need the mass of the knife and the height from the sink to the ceiling, therefore calculating the energy required for the knife to reach the ceiling. From there I need to calculate the damping effect of both the knife (difficult with an irregular shape made from a mixture of materials) and the stainless steel sink. This will give me the impact force required. I am unsure as to whether, given the required force of the throw, gravitational acceleration and/or terminal velocity would come into consideration.

    Can anyone help me as to where I proceed from here?
  2. jcsd
  3. Dec 11, 2013 #2


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    You will have a hard time trying to prove this physically impossible/implausible.
    Steel has pretty high elasticity, so an elastic bounce from the steel sink could be possible.
    It takes very little energy to throw a knife high enough to hit a ceiling, a vigorous throw down at the sink could easily result in an implausible bounce high enough to hit the ceiling.
    A good pitch is about 50mph, about 80ft/sec, so the transit time to the ceiling would be less than a fifth of a second, unless you're living in an old house with really high ceilings. G force is a deceleration of 32ft/sec/sec, so the knife will not slow much if it bounces straight up. Obviously the knife would slow from the bounce and the direction might be more horizontal than vertical, but there is no physical reason it could not bounce from the sink to the ceiling.
  4. Dec 11, 2013 #3
    Thanks for the reply. I was thinking the damping effect of the knife would be more significant than that of the sink (although given the sink is quite thin and suspended it would absorb much more energy than other steel structures - the energy released as sound is an indicator of this).
  5. Dec 12, 2013 #4


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    Chances are the collision would be fairly inelastic (much of the energy is dissipated), yes, but you'll have a very difficult time proving it. There's probably some orientation in which the knife could hit where it would retain most of its energy, and cause an implausible (but not impossible) bounce into the ceiling.
  6. Dec 12, 2013 #5


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    You could challenge her to repeat the trick. That would shut her up, I think. Is there a hole in the ceiling and, if so, does it match the type of knife you are alleged to have thrown?
  7. Dec 12, 2013 #6
    Keeping in mind that it has to be proven "beyond reasonable doubt", implausible should be sufficient. As I said, this is in addition to other evidence (i.e. not being in the country when the knife was broken).
  8. Dec 12, 2013 #7
    I did think about attempting the feat on video to show how difficult it would be. There is apparantly a hole in the ceiling but I had been gone from the house for 1 year when the allegations were made and the incident was from months before I left. Plenty of time for evidence to be fabricated.
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