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I have been trying to find an answer to the question below, but never found any of the answers in google search to be satisfactory.

I know the bullet is lighter in mass as compared to stone. Maybe the bullet exerts a smaller force on the window glass than the stone or may be the stone has a larger area of contact with window, but I cannot relate any of these facts to some law in Physics.

Question

Why does a bullet fired from a gun make a clean hole with minimum cracking/shattering in a glass window but a stone thrown at a similar window shatters the glass? I am assuming that the glass is not bullet proof but just regular window glass.

Sunil
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  • Possible duplicates: http://physics.stackexchange.com/questions/132943/can-i-throw-a-stone-through-my-window-without-breaking-the-glass, http://physics.stackexchange.com/questions/136030/can-a-bullet-shot-through-a-glass-make-a-clean-hole – Yashas Feb 23 '17 at 06:25

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There's something called the "Impact" or "Impulse" factor basically a measure of force multiplied by the time of application of force A bullet has very less impulse factor due to its time of contact, but has enough momentum to break the glass. A stone on the other hand has huge momentum and has enough contact time with the glass for its force to be spread around the glass from its point of contact, thereby shattering it starting from the point of contact.

Try the exact same thing with a small stone, it doesn't happen due to less momentum but you maybe able to see cracks from the point of contact, due to the spread of the impact force.

Example : Easier to cut a piece of a fruit with a knife than a spoon . Also please note, it again depends on the kind of glass as well.

Gary Andrews30
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  • So is the momentum of bullet higher than momentum of stone in this case? – Sunil Feb 23 '17 at 06:33
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    @Sunil The impulse of the bullet is less than that of the stone. J=F*dt But, in case of a bullet, dt is very small due to its high speed. Hence it is able to impart its momentum to only a small area thus creating a neat hole. But the stone has a velocity much less than the bullet. Thus, dt is greater. It gets more time to impart its momentum and that too over a larger area. This makes the glass shatter. – Gary Andrews30 Feb 23 '17 at 06:38
  • That makes sense. It seems the magnitude of impulse determines the destructive effect. – Sunil Feb 23 '17 at 06:44
  • Probably the force exerted on glass by bullet and stone are equal since glass breaks at a certain force on it, and we know glass is pierced by stone as well as bullet. – Sunil Feb 23 '17 at 06:47