Penetrating dense objects at high velocity

1. Nov 24, 2015

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? :)

2. Nov 24, 2015

Staff: Mentor

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.

3. Nov 24, 2015

rjbeery

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.

4. Nov 24, 2015

Staff: Mentor

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.

5. Nov 24, 2015

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.

6. Nov 24, 2015

A.T.

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.

7. Nov 24, 2015

Staff: Mentor

Yes, that's what was bothering me.

8. Nov 24, 2015

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.

9. Nov 24, 2015

rjbeery

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?

10. Nov 24, 2015

A.T.

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.

11. Nov 24, 2015

rjbeery

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.

12. Nov 24, 2015

billy_joule

I agree.

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

13. Nov 24, 2015

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.

Last edited: Nov 24, 2015
14. Nov 24, 2015

phinds

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.

15. Nov 24, 2015

nasu

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.

Last edited: Nov 24, 2015
16. Nov 24, 2015

Staff: Mentor

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.

17. Nov 24, 2015

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.

18. Nov 25, 2015

davenn

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 chasers

Dave

19. Nov 25, 2015

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.

20. Nov 25, 2015

rjbeery

You're really gonna 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...