Surprising Physics: Falling Flat Boards & Chimneys

[Integral]

Russ let me point out that early in this thread I conceded chimneys, see post #3 . The title of the thread is “breaking chalk” . I have further specified; dropped from “several feet” I will refine that to 36” +/- 12”(1m+/- 30cm with zero initial velocity, I believe that this falls into the “several feet “ category. So that should rule out chimneys right there.


Beyond that I claim that your photos do NOT disprove my conjecture. and indeed even support my claim. Note that in the first 3 pictures the top of the chimney is at the level of the top wires, the first break occurs in frame 5, in frame 6 the chimney tip has dropped significantly look at the power lines. It is not a rotation but a direct vertical fall. We see piece 1 in frame 5, in frame 6 we see piece 2. Frames 7 and 8 show the last major unit and the .14 This is not exactly high speed video and, by the way, your video shows nothing of interest, it is impossible to tell how the chimney breaks. Your other photos are inconclusive. I say that your second video the chimney is fall directly in line with the camera, it is impossible at this angle to judge when and where the chimney breaks. Rather then make a claim, I will simply say, I can't tell.

Note in post #9 that I did allow that difference in length was not ruled out.

Moonbear has brought in density as a valid parameter, that, with the radius to length ratio will be some of the critical factors.


Indeed the chalk will shatter due to impact, with the shock wave traveling though the clock at speed of sound in chalk. What is the speed of sound in chalk?

 

[zanazzi78]

I can't really be bothered to read all the comments in this thread (since i`ve only just got here!) but it did give me a dam good reason to break a whole box of chalk and realse some of the days tention!

and yes they all broke into 3 pieces and some dust!

 

[BobG]

Is "several feet" important?

If so, then doesn't this seem like simple coincidence?

Does the length or the width of the chalk matter?

I think it has to be dropped from about π feet.

I've done this accidentally before, it broke into 2 (not 3) big pieces, ~5 small-ish pieces and some very fine dust... but I suppose it matters on the kind of chalk? For example, whether or not it's very thick or pencil-like, or the 'dustless' kind, etc.?

Unless I am wrong--and I am never wrong--that must have been one of the boxes of chalk designed by the Soviets during the Cold War, which normally breaks into 2.7182818 pieces.

I think it's more likely that rachmaninoff dropped the chalk during a time when he was shorter.

I stood on my counter and threw colored pieces of chalk from pie feet high and found they broke into about 8.54 pieces. That's an approximation since I had to try to count them while my crying daughter was scooping up as many pieces as she could, while the pet cat chased and batted stray pieces around the floor, and while my wife shrieked in horror when she saw me standing on the counter next to her pie. I would do more testing, but, unfortunately, I'll living in a car for a few days.

 

[Integral]

I think it has to be dropped from about π feet.


I think it's more likely that rachmaninoff dropped the chalk during a time when he was shorter.

I stood on my counter and threw colored pieces of chalk from pie feet high and found they broke into about 8.54 pieces.

You had to stand on a counter to get \pi feet! You must be very vertically challenged!

That's an approximation since I had to try to count them while my crying daughter was scooping up as many pieces as she could, while the pet cat chased and batted stray pieces around the floor, and while my wife shrieked in horror when she saw me standing on the counter next to her pie. I would do more testing, but, unfortunately, I'll living in a car for a few days.

I know that the price of science is high.. I appreciate your efforts.

Humm... I did not specify color.. wow.. yet another parameter!

 

[BobG]

You had to stand on a counter to get \pi feet! You must be very vertically challenged!

No, not pi feet high. pie feet high!

pie should always be rounded and never squared.

 

[Integral]

No, not pi feet high. pie feet high!

pie should always be rounded and never squared.

Oh... now it is clear, tell me, did your wife object to pie tracks arcoss the floor when you were done?


I have always pondered the obvious error in the statement pie are square!

 

[honestrosewater]

So, without using big words, can anyone tell me if you think the places where the chalk breaks are determined by inconsistencies in the structure of the chalk (it breaks where it's weakest) or inconsistencies in the forces it undergoes (energy becomes more concentrated in some areas) or a combination of both or other factors?

 

[cronxeh]

I think the chalk is too strong to start breaking apart in the air, and it breaks right when it hits the repulsive forces on the ground. The forces are not equally distributed due to the structure of the chalk and depending on the angle, force of collision, area and position of the impact, chalk structure and chemical makeup, it will break along the stress points

 

[PerennialII]

... yeah, it'll be a complex display of fracture considering the 'macroscopic' large cracks appearing in sequence, the smaller 'spalling' etc. with all the redistribution of stresses during the fracture process.

 

[Integral]

In my view I see the shock wave from the impact traveling through the chalk at the speed of sound in chalk and interacting with the stresses induced by the sudden greater then g acceleration of the still falling tip.

Of course this assumes that one end of the chalk strikes first causing a rotation of the stick about that end. Of course the energy of the impact will have a major effect, thus I am restricting this to falls from about \pi feet.

 

[Moonbear]

What flooring material does it have to drop onto? Concrete, tile, linoleum, wood, carpet? Especially wood and carpet will likely cushion the fall and lead to a different fracture pattern than on concrete or tile.

 

[εllipse]

What if a piece of chalk drops in the forest, and there's no one there to see it break?

 

[Integral]

What flooring material does it have to drop onto? Concrete, tile, linoleum, wood, carpet? Especially wood and carpet will likely cushion the fall and lead to a different fracture pattern than on concrete or tile.

My experiments where done on hard classroom floors. I cannot say anything about carpet or cushioned surfaces.

Do I need to specify the brand of tile?

 

[BobG]

My experiments where done on hard classroom floors. I cannot say anything about carpet or cusioned surfaces.

Do I need to specifiy the brand of tile?

Don't do it on carpets - it'll land butter side down.

 

[brewnog]

So, without using big words, can anyone tell me if you think the places where the chalk breaks are determined by inconsistencies in the structure of the chalk (it breaks where it's weakest) or inconsistencies in the forces it undergoes (energy becomes more concentrated in some areas) or a combination of both or other factors?

Certainly not because of any inherent weakness in the chalk. I'm going to use some big words now, because I'm qualified to use them!

When thinking about this problem, we assume that the chalk is isotropic (material properties are the same in any direction), and homogeneous (material properties are constant at all locations within the material).

While there may indeed be some weaknesses in the chalk, and it's impossible to guarantee that there aren't, on a macro scale the chalk is the same at any point along its length.

 

[Integral]

Certainly not because of any inherent weakness in the chalk. I'm going to use some big words now, because I'm qualified to use them!

When thinking about this problem, we assume that the chalk is isotropic (material properties are the same in any direction), and homogeneous (material properties are constant at all locations within the material).

While there may indeed be some weaknesses in the chalk, and it's impossible to guarantee that there aren't, on a macro scale the chalk is the same at any point along its length.

Agreed, those are certainly underlying assumptions. Any micro cracks could completely change the out come!

 

[Integral]

Don't do it on carpets - it'll land butter side down.

Humm... perhaps the butter would reduce the screeching of chalk on board?

 

[brewnog]

Agreed, those are certainly underlying assumptions. Any micro cracks could completely change the out come!

Ahh yes, I wasn't merely informing HRW, I was instructing her on how to introduce SCFs into your chalk collection to make the sticks shatter into 7 pieces instead...

 

[honestrosewater]

Ahh yes, I wasn't merely informing HRW, I was instructing her on how to introduce SCFs into your chalk collection to make the sticks shatter into 7 pieces instead...

Hm, I should have said no acronyms either. (System Control Factors?)

I thought it made more sense that Integral's chalk was partially broken before the drop. It could easily happen during shipping and handling. But fine, you're the experts.
So what types of stess are you guys talking about? Can you infer anything about the types and locations of the stresses from the locations and characteristics of the fractures?

 

[brewnog]

Hm, I should have said no acronyms either. (System Control Factors?)

You got the F right! Stress Concentration Factors. Things like cracks, corners, or sudden changes in geometry will concentrate the stresses, so that they are much higher around those areas. Think of Sellotape being easier to tear if you put a nick in the edge.

I thought it made more sense that Integral's chalk was partially broken before the drop. It could easily happen during shipping and handling. But fine, you're the experts.

Absolutely, and damage would indeed make the chalk break more easily. It needn't even be damage caused by transport or handling, - many manufacturing processes leave imperfections in the product which can cause local weaknesses. However, since we're observing that the chalk breaks at roughly the same places each time, it's unlikely that this is caused by damage.

 

[honestrosewater]

Absolutely, and damage would indeed make the chalk break more easily. It needn't even be damage caused by transport or handling, - many manufacturing processes leave imperfections in the product which can cause local weaknesses.

Yeah, but I didn't feel like looking up how chalk is made. Does anyone already know? I'll check later when I have time to kill.

However, since we're observing that the chalk breaks at roughly the same places each time, it's unlikely that this is caused by damage.

Oh, I didn't know they all did break in the same places, but I thought of that too. If you set a box of chalk on a table and gave it a "karate chop"- you know what I mean?- wouldn't that usually cause similar damage to all or many of the pieces? The "karate chop" could be caused by the corner a heavy box falling on it.

 

[brewnog]

Yeah, but I didn't feel like looking up how chalk is made. Does anyone already know? I'll check later when I have time to kill.

I would imagine it's just compressed in a mould. Part of me thinks that it could be extruded, before being chopped up, but at the moment I'm tempted to say that it's just moulded. Either way, there are no inherent flaws as a direct result of manufacturing.

Oh, I didn't know they all did break in the same places, but I thought of that too. If you set a box of chalk on a table and gave it a "karate chop"- you know what I mean?- wouldn't that usually cause similar damage to all or many of the pieces? The "karate chop" could be caused by the corner a heavy box falling on it.

Yeah, but I think that Integral's spent so many hours on the empirical side of his theory that it's probably more general than just one box of chalk. I imagine him to have sourced different brands of chalk from all over the world, under strictly controlled conditions, before testing them all to destruction in his secret underground lair.

 

[honestrosewater]

Yeah, but I think that Integral's spent so many hours on the empirical side of his theory that it's probably more general than just one box of chalk. I imagine him to have sourced different brands of chalk from all over the world, under strictly controlled conditions, before testing them all to destruction in his secret underground lair.

Hm, I guess his secret underground lair isn't so secret anymore.

 

[BobG]

Hm, I guess his secret underground lair isn't so secret anymore.

During dry spells, you can just follow the trail of chalk dust.

Oh, wait. He lives in Oregon.

Integral, dry spells have nothing to do with witches. They're prolonged periods without rain.

 

[honestrosewater]

Integral, dry spells have nothing to do with witches. They're prolonged periods without rain.

I thought witches were women who practiced sorcery and had magical powers.?

 

[BobG]

I thought witches were women who practiced sorcery and had magical powers.?

Which witch? The ones that wear black hats or the sand witches that hang around with dust devils?

 

[Janus]

During dry spells, you can just follow the trail of chalk dust.

Oh, wait. He lives in Oregon.

Integral, dry spells have nothing to do with witches. They're prolonged periods without rain.

Oh we know what those are here in Oregon, sometimes we get them that last as long as a WEEK!

 

[honestrosewater]

Which witch? The ones that wear black hats or the sand witches that hang around with dust devils?

It could be either- I only know they always carry brooms.

 

[honestrosewater]

Oh we know what those are here in Oregon, sometimes we get them that last as long as a WEEK!

Do yours carry brooms?

 

[Integral]

I would imagine it's just compressed in a mould. Part of me thinks that it could be extruded, before being chopped up, but at the moment I'm tempted to say that it's just moulded. Either way, there are no inherent flaws as a direct result of manufacturing.



Yeah, but I think that Integral's spent so many hours on the empirical side of his theory that it's probably more general than just one box of chalk. I imagine him to have sourced different brands of chalk from all over the world, under strictly controlled conditions, before testing them all to destruction in his secret underground lair.

My testing was done sporadicly over my 15 years in academia, if front of many different black boards in many different class rooms. Most of the chalk I have dealt with has been of very similar dimensions, but I have allowed dimension as a critical parameter.

Now as for tracks into the secret lair, it is not clear? Are those \pi tracks from Bobs \pi feet or dusty chalk track?

I am sure that broom carrying lady was just the janitor sweeping up the chalk tracks (which are much easier to clean up then \pi tracks) and not a witch!

As I write, I notice that something strange is happening outside, there is some unusual form of energy pouring through my windows, and the clouds all seem to be blue?
Janus, what is that big yellow thing in the sky?