Our neigbours glass windows has allegedly been shattered by neibouring kids. The question is whether the stone came from below the window thrown by someone standing on the ground or above the window from someone standying on the adjacent roof. Is it possible to determine the position of the thrower when looking at the fractured glass?
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I suspect it cannot be determined. Unless the stone was a very fast projectile making a small directional hole it would cause wide shattering since it transferred most perpendicular momentum to the pane (see this chapter in a forensics textbook). Wide shattering happens because there is a deformation inwards but enough time to transfer all the momentum.
Now, the angle the stone made with the glass surface does not matter since the only thing that matters is the perpendicular momentum. I suspect the lateral momentum (along the surface) of the rock is not transferred well (smooth hard surface, shear resistant) so the only thing that can be figured out is the overall angle, not whether it was from above or below (as Trula commented, parabolas from below can hit on the descending arc).
If this had been a CSI episode I would expect the next scene to be a montage of fruitless but fun rock-throwing at glass panes, and finally Grissom pointing out the above to the team... followed by a "wait! there is grass on the rock!" or some other piece of evidence of the rock origin.
Anders Sandberg
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