Penetrating dense objects at high velocity

[rjbeery]

Movement is relative but I find it counter-intuitive that a slab of concrete moving at 150+ mph would be impaled by stationary piece of wood.

Similarly, if we throw a palm tree at a single piece of straw it is counter-intuitive to believe that the straw would penetrate the tree in any way! (See: http://mythbustersresults.com/episode61)

Is there something else going on here that would make these alternate experiments behave differently (e.g. a surrounding atmosphere)? Or do we actually predict the same result despite what our intuition says*?

*Or perhaps your intuition differs from mine? :)

 

[DrClaude]

What is relative is the frame in which you chose to describe the motion.

I think that the difficulty you have comes from the fact that your are imagining a reversed situation that is not equivalent. A piece of wood hitting a fixed block of concrete is not the same as a flying piece of concrete hitting a fixed stick. For one, the moving piece of wood at a certain speed has much less kinetic energy than the block of concrete moving at the same speed, such that less energy needs to be dissipated to stop the motion.

 

[rjbeery]

What is relative is the frame in which you chose to describe the motion.

I think that the difficulty you have comes from the fact that your are imagining a reversed situation that is not equivalent. A piece of wood hitting a fixed block of concrete is not the same as a flying piece of concrete hitting a fixed stick. For one, the moving piece of wood at a certain speed has much less kinetic energy than the block of concrete moving at the same speed, such that less energy needs to be dissipated to stop the motion.

I see that you agree that the conclusion seems odd, but I'm afraid your explanation isn't correct. An object doesn't "have" kinetic energy in any absolute sense. Galileo would insist that the concrete and the wood would react in the same manner regardless of which is apparently moving.

 

[DrClaude]

I see that you agree that the conclusion seems odd, but I'm afraid your explanation isn't correct. An object doesn't "have" kinetic energy in any absolute sense. Galileo would insist that the concrete and the wood would react in the same manner regardless of which is apparently moving.

You're right, my discussion of kinetic energy was incorrect. But the situations still seems to me to be asymmetric. I'll have to think about it some more.

 

[gjonesy]

Consider this for a moment, a standard .223 rifle round you can literally destroy one with a set of pliers. The copper jacketed lead bullet is soft. But load it in a rifle and fire it and it has the energy to penetrate a concrete block. Its weight is around 2 grams, but its traveling at m3. But of course both are destroyed in the process. I have no problem believing that if you mounted a concrete slab of the same size to a rocket sled and fired it at a shard of wood that you'd get a similar result.

 

[A.T.]

Movement is relative but I find it counter-intuitive that a slab of concrete moving at 150+ mph would be impaled by stationary piece of wood.

You mean a slab of concrete attached to a planet moving at 150+ mph vs. a stationary piece of wood? There is no difference to stationary slab of concrete attached to a planet vs. piece of wood at 150+ mph.

My intuition has more problems with the thin pointy tip being barely blunted, and the broken concrete debris not being thrown off from the top.

 

[DrClaude]

You mean a slab of concrete attached to a planet moving at 150+ mph vs. a stationary piece of wood? There is no difference to stationary slab of concrete attached to a planet vs. piece of wood at 150+ mph.

Yes, that's what was bothering me.

 

[nasu]

It's more likely that the concrete slab already had a hole and the wood just got stuck in it. Maybe cracked the concrete a little, as it was jammed in.
You can actually see another hole in that slab, a little lower and to the right. I don't suppose that was also made by another piece of wood.

 

[rjbeery]

It's more likely that the concrete slab already had a hole and the wood just got stuck in it. Maybe cracked the concrete a little, as it was jammed in.
You can actually see another hole in that slab, a little lower and to the right. I don't suppose that was also made by another piece of wood.

But the boys at Mythbusters showed a similar effect when they embedded a piece of straw a quarter-inch into a palm tree!

And the wood was moving parallel to the Earth's surface, A.T., so it's difficult for me to attribute any asymmetries to that. Are you claiming that the concrete would simply smash the wood to bits if it were moving at 150+ mph towards the wood but neither were attached to the Earth?

 

[A.T.]

Are you claiming that the concrete would simply smash the wood to bits if it were moving at 150+ mph towards the wood but neither were attached to the Earth?

If neither is attached to the Earth, then you have different situation than when one is attached to the Earth. So it's not just a matter of relative movement. That's what I'm saying.

 

[rjbeery]

If neither is attached to the Earth, then you have different situation than when one is attached to the Earth. So it's not just a matter of relative movement. That's what I'm saying.

Well you're right in pointing out that the concrete is "attached" to the Earth, but it's still difficult for me to reconcile why that would make a difference or why the wood did what it did, regardless.

 

[billy_joule]

My intuition has more problems with the thin pointy tip being barely blunted, and the broken concrete debris not being thrown off from the top.

I agree.

I think the image shows something that could only occur if some pre existing defect/void in the kerb is present.

 

[gjonesy]

VERY Plausible!

There could have been a hole in the curb to allow for drainage. Then the wood shard could have acted like a big wooden wedge. The wood, actually from the looks of it may be cedar. Cedar is a dense hardwood, very tough and durable. The wood would have acted like an inclined plane and (force multiplier), the concrete would have had two weaker points on either side of the hole. When you strike a piece of obsidian rock when flint napping the force always travels at an angle from the impact point. That would acount for the pattern of fracturing in the concrete.

 

[phinds]

Cedar is a dense hardwood, very tough and durable.

Wrong on two of three and the two significant ones at that. Cedar is not a hardwood, it is a light-weight softwood and not very tough as woods go. It is durable in situations (moisture) where other woods will rot but that's irrelevant to the issue at hand. "Cedar" encompasses quite a few species and most of them you can very easily dent with your fingernail. None of them are what you would really call strong relative to other woods such as oak.

 

[nasu]

But the boys at Mythbusters showed a similar effect when they embedded a piece of straw a quarter-inch into a palm tree!

It does not follow that this is what happened here.
When you have a simpler and more likely mechanism (see second hole) the Mythbuster experiment is not necessarily relevant.
And the straw in wood is quite dissimilar to wood in concrete. I can stick a toothpick in a piece of soft wood without any special conditions. Just by hand. It won't work in concrete, though.

PS. It looks like one of those old "amazing facts" that circulate the net and is attributed to various places or times.
Like it was the "huge" planet Mars in the sky (larger than the full moon) which shows up every few years.
https://www.reddit.com/r/WTF/comments/2sj25m/wood_driven_by_a_tornado_through_concrete/

 

[Dale]

As others have mentioned, I am skeptical of the concrete and wood photo, so let's stick with the mythbusters scenario.

Similarly, if we throw a palm tree at a single piece of straw it is counter-intuitive to believe that the straw would penetrate the tree in any way! (See: http://mythbustersresults.com/episode61)

Consider a variant of this scenario where the experiment is performed in a train car at rest. The result will be the same. Now consider the scenario with the train car moving on a smooth level track. The result will be the same. Now, consider the scenario with the train car on a bullet train moving so fast that the straw is at rest after being fired. The result will be the same.

 

[nasu]

It does not matter how much energy the colliding objects have (which is dependent on the reference frame) but how much of it is used (lost) during the collision. This determines the effect of the collision and should depend only on the relative speed.

 

[davenn]

My intuition has more problems with the thin pointy tip being barely blunted, and the broken concrete debris not being thrown off from the top.

regardless of if this particular pic is real or not ...
its nothing out of the ordinary that I haven't already seen in may yrs of stormchasing and seeing pic's from fellow chasersDave

 

[jbriggs444]

There are at several asymmetries that have not been mentioned for this particular collision. Concrete is brittle. Wood is resilient. The collision was side-on into the concrete but end-on for the wood.. The collision would have been aligned with the grain in the wood.

 

[rjbeery]

As others have mentioned, I am skeptical of the concrete and wood photo, so let's stick with the mythbusters scenario. Consider a variant of this scenario where the experiment is performed in a train car at rest. The result will be the same. Now consider the scenario with the train car moving on a smooth level track. The result will be the same. Now, consider the scenario with the train car on a bullet train moving so fast that the straw is at rest after being fired. The result will be the same.

You're really going to explain relativity, like that wasn't explicitly mentioned in the OP? Anyway I think nasu's link nailed it: the wood is sitting in a PVC drain hole through the concrete! You can actually see a bit of the pipe. Makes me feel much better about this...

 

[Dale]

You're really going to explain relativity, like that wasn't explicitly mentioned in the OP?

A confusion about relativity was mentioned in the OP. So, yes it is good to explain relativity to clear up the confusion and hopefully improve your intuition.

 

[rjbeery]

A confusion about relativity was mentioned in the OP. So, yes it is good to explain relativity to clear up the confusion and hopefully improve your intuition.

I see. Thanks for your attempted contribution...

 

[Dale]

I see. Thanks for your attempted contribution...

You are very welcome!

 

[nasu]

There are at several asymmetries that have not been mentioned for this particular collision. Concrete is brittle. Wood is resilient. The collision was side-on into the concrete but end-on for the wood.. The collision would have been aligned with the grain in the wood.

The main asymmetry is that the concrete had pre-existing holes in it and the wood did not.

 

[gjonesy]

Wrong on two of three and the two significant ones at that. Cedar is not a hardwood,

You are right I was mistaken. But every time I've purchased it, its been in the same section as oak and hickory. I'll concead that it's classified as a soft wood. However as far as its durability we have to disagree. Ask any farmer that owns a pasture with a barbed wire fench or a tobacco barn what kind of wood is used in them and most often you will hear the response Cedar. Once Cedar weather's it turns white or ashen gray and is very tough and hardly rots. They use it because it last a very long time.

 

[phinds]

You are right I was mistaken. But every time I've purchased it, its been in the same section as oak and hickory. I'll concead that it's classified as a soft wood. However as far as its durability we have to disagree. Ask any farmer that owns a pasture with a barbed wire fench or a tobacco barn what kind of wood is used in them and most often you will hear the response Cedar. Once Cedar weather's it turns white or ashen gray and is very tough and hardly rots. They use it because it last a very long time.

AS I SAID, it is durable in terms of moisture, which has zero relevance to this thread. Turning white/gray is a natural result of UV and oxygen destroying the pigments out of the wood and does not make the wood any stronger. Besides, the piece in the pic is red, not white/gray.

 

[gjonesy]

Concrete is brittle. Wood is resilient.

Excellent point. Wood tends to flex and deform absorbing energy while staying intact till its over burdened and breaks. Concrete on the other hand is very hard dry inflexible and as you said brittle. It's extremely tough until you hit a defect, internal void or air pocket. What comes to mind is a machine I've seen in some concrete production that basically vibrates a concrete slurry mixture to get air bubbles out of concrete and sement preforms. Which make it much more durable. That concrete curbing was probably pured on the site, had drains placed in it to prevent water from pooling behind the curb in heavy rains. All of this could have contributed to the picture we see. Great observation.

 

[gjonesy]

AS I SAID, it is durable in terms of moisture, which has zero relevance to this thread. Turning white/gray is a natural result of UV and oxygen destroying the pigments out of the wood

AND AS I SAID. you were right its classified as a soft wood. And the sun doesn't shine on THE INSIDE of wood. And that's the part you are seeing in the picture not the outside. The reason Cedar is tougher then pine and other conifers is its structure. If you look at a Cedar blank and a pine blank side by side you will see a big difference in the grain pattern, pine and other (softer) woods the grain is wider and more space out. In cedar its tighter in my experience with it. It's definitely harder then pine. If you've ever tried to chop dried cedar it with an axe you'd see what I mean. And besides it's ability to absorb energy is because it is soft enough that it flexes. Which makes it very tough. The reason you don't see martial artist trying to break wood like "Cedar" is because of the grain. It's tougher absorbs more energy and is harder to break. I have personal experience in farming and woodworking and martial arts. So its not so irrelivent.

 

[anorlunda]

In the case of tornadoes, there is more going on; namely low air pressure.

I've heard that the low pressure can cause the fibers of a tree to briefly open up enough to be penetrated by a piece of straw. That may be myth, but the real point is that, yes there may be lots of other things going on to explain strange outcomes.

 

[nasu]

Yes, but first you should be sure that the outcome is "strange".

 

[Beach Geek]

Few personal observations...
1. Pine needles can penetrate more than 0.50" into Chicago brick. Source: my house in 1979 hurricane Fredric.
2. Cedar is not stronger pine. Take a 2x4 of each and hit it on the endgrain with a 22oz framing hammer, cedar will crush and pine will have a dent(lodgepole) or barely a mark (southern yellow).
3. Concrete comes in different strengths from 1500psi to 7000psi mix. Curbs vary, but 2500-3500psi would be reasonably.

I agree with earlier comments about a drain hole or at least a flaw where the wood is.

PS. Never thought I'd have to "shave" my house. ;)

 

[Sturk200]

If you imagine suspending the wood by a piece of thread or fishing line or something and then whacking it at adequate speed with a concrete slab, it doesn't seem particularly counter-intuitive to think that the wood would be made to penetrate the rock. My intuition only has trouble when I imagine having the wood like resting on the ground or something and then hurling the rock at it. The trouble is that the wood in that case would always break before it had a chance to pierce the rock. But if you suspend it in such a way that there's nothing for the wood to break against, the penetration seems much more plausible.

So maybe the asymmetry is that the concrete is sturdy enough so as not to be destroyed by the impact, whereas the wood must be given special treatment so as not to be destroyed.

 

[gjonesy]

2. Cedar is not stronger pine. Take a 2x4 of each and hit it on the endgrain with a 22oz framing hammer, cedar will crush and pine will have a dent(lodgepole) or barely a mark (southern yellow).

It really depends on what you are talking about when you say "strength" and most importantly when you say "cedar", there is western red cedar and Canadian or Alaskan cypress which is called cedar. Pine has resin channels where as cedar doesn't, there for its harder because the grain is tighter.

http://woodmonsters.com/strength-chart.html