# The Naked Scientists Forum

### Author Topic: Fast hits solid - what happens?  (Read 3890 times)

#### peterclarke

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##### Fast hits solid - what happens?
« on: 24/11/2005 20:30:57 »
What happens if a body, such as a fully loaded aeroplane, hits a stationary object, such as a skyscraper, at full speed? What is the physics? Common sense might suggest that the plane made of light materials is instantly destroyed when it hits the heavy steel and concrete building. But does the high velocity of the plane change things? Does the plane cut through the building?

You may detect hints of  9/11 in my question - but I am not interested in any conspiracy theories - just the science.

Peter

#### ukmicky

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##### Re: Fast hits solid - what happens?
« Reply #1 on: 24/11/2005 20:55:15 »
It would depend on what it was made from, how fast it was moving and how dense its structure was.

With 9/11 a piece of a plane can be seen exiting the building via a window on the opposite side to which it hit, but that was because there must of been a relatively unobstructed route through the building for that part of the plane.

However i actually saw a recording on TV about ten years ago showing an F16 fighter flown by remote control at full speed along a track way into a large solid block of concrete for some test. The result was a few tons of flattened metal,the plane just collapsed in on itself. The concrete block survived with only a few marks,even the engines folded up.

Ok it wasn't flown as it was tethered to a rail. but you understand what i mean.
Michael
« Last Edit: 24/11/2005 21:08:19 by ukmicky »

#### peterclarke

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##### Re: Fast hits solid - what happens?
« Reply #2 on: 24/11/2005 21:33:56 »
Hmmm... I am still not clear about this (quite normal for me!) - does the F16 example (a flying train???) imply that given the relationship between the mass of the plane and the mass of concrete the result will be similar at any speed above a certain level?

Peter

#### Simmer

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##### Re: Fast hits solid - what happens?
« Reply #3 on: 24/11/2005 22:52:02 »
No, when a plane hits a concrete block and stops it passes its momentum onto the block.  The greater the speed and/or weight of the aircraft the greater the momentum (p=mv).  At a high enough velocity the block would move or break.

#### Malus32

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##### Re: Fast hits solid - what happens?
« Reply #4 on: 25/11/2005 00:30:23 »

Doesn't it also depend on the weight of the plane? If some
casual glider crashes into the block, compared to a large jumbo jet,
wouldn't it make a difference?

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#### ukmicky

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##### Re: Fast hits solid - what happens?
« Reply #5 on: 25/11/2005 02:32:43 »
i could be wrong but I would of thought what happens  would depend on two things

What material the projectile is made from, and how heavy and immoveable the concrete block was.

Take one jet f16 fighter and fly it into a 100ton block of concrete at supersonic speeds and you will end up with a mangled mess of aluminium nothing else, I doubt very much the block of concrete would even move an inch

In the end the f16 airframe is light and made out of aluminium, put enough energy into its forward momentum and it would disintegrate, the bonds holding the aluminium atoms together would rip apart and heat up leaving nothing more than dust and  a bit of molten aluminium laying on the floor and this would probably happen well before the point of moving the concrete block.

Make your planes nose out of titanium and it would be a different story as titanium has tighter and stronger atomic bonds which are stronger than the bonds holding the concrete atoms together and so it would probably force its way partly through

Michael

#### DoctorBeaver

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##### Re: Fast hits solid - what happens?
« Reply #6 on: 25/11/2005 18:12:52 »
quote:
However i actually saw a recording on TV about ten years ago showing an F16 fighter flown by remote control at full speed along a track way into a large solid block of concrete for some test. The result was a few tons of flattened metal,the plane just collapsed in on itself. The concrete block survived with only a few marks,even the engines folded up.

I've seen that. If I remember correctly, a large proportion of the F16 actually disintegrated completely. I think it was part of a test to do with construction of atomic facilities & their resistance to attack

#### DoctorBeaver

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##### Re: Fast hits solid - what happens?
« Reply #7 on: 25/11/2005 18:15:49 »
The mass & the speed of the object are major determining factors. A freight train hitting a wall at 60mph would do a lot more damage than the same train hitting the wall at 5mph, and a LOT more than a family hatchback hitting the wall at an equal speed.

#### Solvay_1927

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##### Re: Fast hits solid - what happens?
« Reply #8 on: 26/11/2005 02:47:23 »
For what it's worth, here's my (amateur-wannabee-physicist) take on it:  you're all correct!

The most fundamental law of physics is the law of conservation of energy.

The kinetic energy of the plane (calculated as (m.v^2)/2) has to go somewhere, and if most of it is imparted to the building, then the amount transferred increases with the plane's mass (m) and/or velocity (v).

So a heavier plane will do more damage than a lighter plane (travelling at the same speed), and a faster plane will do more damage than a slower one (of the same mass).

But Michael's right when he says that it's also highly dependent on the structures and materials involved, because a plane which crumples easily will allow more of the kinetic energy to be dissipated before it's imparted to the building. (Dissipated in the form of heat, including sound.)

Think about when you're driving your car at high speed.  If a stone (even a small, light one) hits your winsdscreen you'll probably have to go and buy a new windscreen.  But if a large insect (weighing exactly the same as the stone) hits your windscreen, all you have to do is turn on the wipers.  The only difference is that the stone doesn't crumple (so it transfers most of its energy to the windscreen), whereas the insect has lost most of it's energy to the surrounding air by the time it's finished going "splat".

Likewise, the structure/material of the building obviously matters too.  If your windscreen is made of rubber rather than glass (transparent rubber, I'd suggest!) then the energy would get dissipated quickly throughout the windscreen (in the form of bending and stretching).

Incidentally, when driving along a road with a "loose chippings" sign, I normally keep the fingers of one hand pressed against the windscreen, to absorb some of the impact of any stones that hit the glass (so taking away some of the energy that would otherwise be trapped in the windscreen and cause it to shatter).
(I've no idea if that actually works or not - it's just something my dad taught me to do many years ago, when he was teaching me how to drive, and old habits die hard! )

#### DoctorBeaver

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##### Re: Fast hits solid - what happens?
« Reply #9 on: 26/11/2005 15:19:01 »
quote:
Incidentally, when driving along a road with a "loose chippings" sign, I normally keep the fingers of one hand pressed against the windscreen, to absorb some of the impact of any stones that hit the glass

I was taught that too. & I also don't know if it actually works. I'm not convinced it would as there is not much "give" in a windscreen. It's not as if your finger would act as much of a shock-absorber.

#### Soul Surfer

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##### Re: Fast hits solid - what happens?
« Reply #10 on: 26/11/2005 18:07:29 »
The important thing to remember is that buildings, like planes are mostly empty space (windows, rooms etc) and not concrete blocks and both collapse under violent forces.  We have all seen many times what happens

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##### Re: Fast hits solid - what happens?
« Reply #10 on: 26/11/2005 18:07:29 »