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Random questions: explain BB gun bullet dent in (regular) glass?

  1. Sep 18, 2010 #1
    Hi all, this has been bugging me so maybe you can help me out.

    In the front window of my house, there is a dent in the glass that looks somewhat similar to this:

    http://www.flickr.com/photos/guerra/2557095422/" [Broken]

    However, there is not hole in the glass. But the shape of the dent is similar. The thing is, I am told that this dent was cause by a bb gun bullet that was shot FROM THE OUTSIDE, yet the dent looks like it was hit from the INSIDE (from a layman's intuition).

    This seems like it could have a physics explanation but I could be wrong. Can somebody explain why this happens?

    The dent from the inside looks like a crater, as in it starts out like a wide circle, and the center of this circle is it's deepest point. Hence why it is puzzzling that the bullet hit on the outside. By nature of my window I cannot examine it from the outside (it's on the second floor, etc.) but it almost appears as those the outside of the glass is not damaged, only the inside.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 18, 2010 #2
    BB strikes outside of glass. Outside of glass is supported against the pressure by inside of glass, passes energy along to inside. Inside of glass is not supported, dissipates energy by breaking.

    A similar effect can be seen in a device called newtons cradle. This is a pendulum like device where energy is transfered from one end of a row of balls to the other with only minimal movement of the balls in between. In your window the BB was like the ball on one end, the missign chip of glass was like the ball on the other end and the unharmed glass was like the balls in the middle.

    Last edited by a moderator: Sep 25, 2014
  4. Sep 18, 2010 #3


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    I have to agree with Speedy on this; the impact was probably just transfered from the outside to the inside. First order of business is to find out who did it and make the bastard pay for a new window. Criminal charges might ensue. A BB gun or pellet gun is no joke. Not only are some of them similar enough in design to be used in armed robberies, but even a very low-power device can kill a human or at least take out an eye. Whoever did it should be taken off of the street before he gets his mitts on a Tec-9 and causes some serious grief.
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