# Why does a rock shatter a glass, but a bullet pass through.

This is a simple enough question, but no one has been able to give me a satisfying explanation. Most people compare the momentum's of the bullet and the rock, and say that rock has a higher mass, so it's momentum is greater and it shatters the glass.

I don't find this a convincing explanation, and the obvious flaw is that the bullet has a much higher velocity, which would make it's momentum atleast of the same magnitude as that of the rock's.

It appears to me that the rock completely transfers it's momentum to the glass, but the bullet only loses a fraction of it's momentum. I can't think of a reason for why this happens.

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It's like using a spoon to cut food versus using a knife. The spoon just smacks the food -- the knife cuts through it and leaves a mark on the plate. The bullet's energy is more focused, like a knife; while the blunt rock is forced to disperse its energy as it is stopped by the glass. It has enough energy perhaps to break through like the bullet, but it isn't focused on an area small enough to do it. This results in most of the glass receiving enough energy to be shattered but not completely pierced.

Probably the bullet doesn't give enough time for the force on the glass to travel more widely before the local area around the bullet is already destroyed and so there is no more force being applied.

A slightly similar experiment is done if you hang a weight by thread then hang another thread off the weight. If you pull the bottom thread slowly the top thread will break, if you pull it fast then the bottom thread will break.

Andrew Mason
Homework Helper
This is a simple enough question, but no one has been able to give me a satisfying explanation. Most people compare the momentum's of the bullet and the rock, and say that rock has a higher mass, so it's momentum is greater and it shatters the glass.

I don't find this a convincing explanation, and the obvious flaw is that the bullet has a much higher velocity, which would make it's momentum atleast of the same magnitude as that of the rock's.

It appears to me that the rock completely transfers it's momentum to the glass, but the bullet only loses a fraction of it's momentum. I can't think of a reason for why this happens.
It is all about speed. The rock will make a small hole too (the size of the rock) if it is going fast enough.

The glass molecules are all connected but it takes a finite amount of time for the glass molecules that are directly hit by the object to cause adjacent ones to move and for that effect to spread out over a larger area. If the object is going slow enough, the glass molecules have time to pull on adjacent molecules before being blasted out of the way by the object. So the impact of the object is spread out over a larger area.

AM

Danger
Gold Member
This whole thread puzzles the hell out of me. I've shot a lot of glass in my time, and it always shattered. The only exceptions were laminates such as automotive windshields. (Even then, the glass parts shattered but were contained by the polymer sandwich.)
It's obvious that I'm missing something here, but I have no idea as to what it is.

Tempered glass will shatter, non tempered you might get the bullet hole. Shooting with a bb gun makes holes in normal non tempered glass, like a house window at least used to have.

A.T.
As already said, it depends on the glass. Here some amazing footage (glass starts at 2:47 & 4:29):

DaveC426913
Gold Member
foggyeyes, think about the magician's trick where they yank a tablecloth out from under a fully set table of dishes.

Yank slowly and the tablecloth has enough time to impart its movement to the dishes. Yank fast enough and the tablecloth goes before it can impart any motion to the dishes

Probably the bullet doesn't give enough time for the force on the glass to travel more widely before the local area around the bullet is already destroyed and so there is no more force being applied.
Probably the first correct answer, I agree.

My answer would be if it's a rock, the glass tries to fight so ends up shattering whole. If it's a bullet, glass knows it has no chance, raises the white flag and makes a neat 'lil hole to let bullet pass.

It's like guy trying to fight a mugger armed with knife ends up dead, vs another guy letting mugger with gun get whatever he wants and ends up alive and probably unharmed.

That doesn't make much sense. What do you mean by, the glass know it's beaten when a bullet is fired at it ?

The logic that the bullet has less time to impart momentum to the glass, seems fairly convincing.

It is because the bullet is smaller and sharper than the rock and travels at a higher speed, so exerts more pressure on a small area of the glass, creating a hole for it to go though. The rock affects a larger area of the glass with a lower pressure, so the whole glass is more likely to shatter. Compare shooting a tomato with a pin to hitting it with a hammer. In the second case it will splatter, in the first, it will not.

DaveC426913
Gold Member
It is because the bullet is smaller and sharper than the rock and travels at a higher speed, so exerts more pressure on a small area of the glass, creating a hole for it to go though. The rock affects a larger area of the glass with a lower pressure, so the whole glass is more likely to shatter. Compare shooting a tomato with a pin to hitting it with a hammer. In the second case it will splatter, in the first, it will not.
Your assertion assumes that the same effect would not occur if the rock had the same cross sectional area as the bullet.

I don't think that size is the major factor here - more accurately, I think size could be factored out, and I think we would still see the same effect.

Take the rock, accelerate it to three times the speed of sound and I think we will see an effect identical as with a bullet.

Also, comparison of hitting a tomato to throwing a stone on a glass window, is wrong. When you hit the tomato with a hammer, I assume you meant to hit it perpendiculary, when the tomato is on some sort of a surface. Doing so would cause the surface the tomato is resting to exert normal force on the tomato itself.

The correct analogue would be to assume that the hammer strikes parallel to the surface on which the tomato rest

Your assertion assumes that the same effect would not occur if the rock had the same cross sectional area as the bullet.

I don't think that size is the major factor here - more accurately, I think size could be factored out, and I think we would still see the same effect.

Take the rock, accelerate it to three times the speed of sound and I think we will see an effect identical as with a bullet.
Yes, but as the rock is much heavier than the bullet, it would have a much higher momentum. I was assuming the momentum was the same. Either way, it is the pressure that counts. If the rock is going at the same speed as the bullet, it will exert a simerlar pressure.

Also, comparison of hitting a tomato to throwing a stone on a glass window, is wrong. When you hit the tomato with a hammer, I assume you meant to hit it perpendiculary, when the tomato is on some sort of a surface. Doing so would cause the surface the tomato is resting to exert normal force on the tomato itself.

The correct analogue would be to assume that the hammer strikes parallel to the surface on which the tomato rest
No, it wouldn't. If you want to be pernickity about it, it would be that the hammer strikes a tomato which is held in place by glue to the inside of a hole in a plank held vertically. The effect would be simerlar. In the glass example, if the glass had a mass comparable to a tomato and wasn't stuck to a frame, the rock would just knock it over, and I'm not sure what the bullet would do.

Perhaps some analogy would be good for understanding. Based on my consideration, when you stab a diaphanous layer, when you do it quickly it is likely to make a hole; when you do it slowly you would mess it up. It is a common phenomena when you try to stab a tube into a milk box.

My explanation would be when speed is low the surface has enough time to propagate the action before it is broken. When it is too fast, the surface locally breaks very fast. Also, when time taken to break the surface more momentum is transferred, since F=dP/dt, ∫Fdt=P.

DaveC426913
Gold Member
Perhaps some analogy would be good for understanding. Based on my consideration, when you stab a diaphanous layer, when you do it quickly it is likely to make a hole; when you do it slowly you would mess it up. It is a common phenomena when you try to stab a tube into a milk box.
Good analogy! I was struggling toward a similar one with drinking cups from fast food joints. Yours is better.

Everyone knows that, to poke the straw into the juicebox, you've got to do it with a fast jab, not a slow push.

^Both of the above posts can again be explained by pressure.

What about the friction and heat generated by the speed of the bullet? I would assume this would help the bullet pass through the glass structure with the heat creating expansion between the molecules. This would be 1000 times greater due to the difference of pressure the bullet applies compared to the rock, pressure being F/A.

I don't know the physics of the event, but armor piercing rounds are hardened steel or tungsten tipped bullets that transfer energy on impact to a small area making it explode from the rapid rise in temperature. Not friction as I think of it anyway, more directly related to the impact.

A local gas station was torn down when I was a kid, and I collected a bunch of glass that was used in the small windows of a rollup door. After a few years of doing nothing with them I acquired bot a wrist rocket type sling shot and a BB gun. Using the glass as targets I found tempered glass pretty much always will shatter, but regular glass would get holes from any fast moving projectile.

Biggest argument against friction, BB's make a hole and often, maybe never, go through the glass, bouncing off instead.

A bullet passing through glass wont cause the bullet to explode because the velocity and material of the bullet exceeds the ability of the glass to hold its molecules together, that would be my understanding.

On the other hand, a hand fired BB would probably be too slow to generate enough momentum or heat to pass through the glass, the angle of impact could also be a factor as a high angle of reflection would probably allow the glass reflect force more efficiently.

Perhaps friction isnt a factor but Im sure heat plays a part in seperating the material which is more brittle then the projectile.

Could be heat from impact, could be some kind of mechanical failure related to the transfer of energy (one of the links at the bottom of the thread talks about similarity to a Newton's cradle, the half dozen metal balls suspended on strings where only the end ball moves on impact).

The point isn't could the bb with sufficient speed pass through the glass, its that the "bullet hole" effect occurs without the BB passing through, which makes a strong argument that the "passing through" is of little effect. I don't think speed is either except in a yes/no does it break a hole in the glass.

Whether the bullet passes through or not is likely a function of the mass of the bullet and the mass of the effected region of the glass (maybe the mass of the divot that pops out, or maybe including something of the immediate area around it). Its a "mostly" inelastic collision, so bullet less than glass mass bounces off, bullet equal to glass mass stops dead, bullet greater than glass mass goes through providing a hole was created in the impact.

Its all quantum physics to me and I dont know enough of that ;)

Its all quantum physics to me and I dont know enough of that ;)
I don't think its that "high end", more likely the mechanical impact argument, as the behavior isn't unique to glass, mud (allowed to dry first of course) does the same thing and its a mechanical mixture. Key to the concept is that the divot is knocked out, not pushed out.

Maybe also important is the speed that an impulse travels through the target. I say impulse, but "sound" might be a good way to think of it. The impulse speed limits the area effected in the target by the speed of the bullet.

The reason a ball bearing can bounce off, but still create a hole in the glass, is because it is the tension wave that breaks glass.

I'm sure you've noticed that glass is much stronger in compression than tension. When the ball bearing hits the surface of the glass in introduces a compression wave which the glass is strong enough to sustain, when this wave reaches a free surface it causes a release (tension) wave, this exceeds the tensile strength of the glass (more specifically it is the interaction of two release waves that causes the glass to fail) causing the glass to break. This process takes time, sufficient time for the glass to stop the bb.

The same process causes mechanical spalling (http://en.wikipedia.org/wiki/Spall).