Crash Physics: Deceleration & Material Properties

[alibongo]

Hi everyone, I'm trying to understand crash physics.

When a moving object collides with a static object, is there always deceleration present at impact?
If the moving object is designed to pass through something else (a bullet or missile) I assume there is very little deceleration if any.

Do the crash physics displayed depend on the material or speeds of the objects involved?
Is there any way a car for example could pass through a brick wall without any deceleration, or breakage?
If the car was capable of traveling at say 300 mph, might it pass through a wall with no deceleration or breakage? Are the properties of the objects and the speeds of the objects at all relevant?

 

[Danger]

Welcome to PF, Alibongo.
There will always be some degree of deceleration, unless the moving object is a neutrino or maybe a high-frequency photon. A freight train hitting a mosquito slows down by an immeasurably tiny amount.
Both the materials and the speed are extremely relevant, as are the shapes of the objects. In fact, those are pretty much the only factors that do matter.
By the way, a properly designed bullet stops completely within the target. If it goes through, a lot of energy is wasted rather than causing damage. That's what hollow-points are all about.

 

[buffordboy23]

Hi alibongo, welcome to PF!

Hi everyone, I'm trying to understand crash physics.

When a moving object collides with a static object, is there always deceleration present at impact?

In the macroscopic world, the answer is definitely yes. Energy must be conserved. For example, if you hear sound from the impact, energy from the system of the two colliding objects loses energy to its environment. As a consequence, some deceleration must occur.

Do the crash physics displayed depend on the material or speeds of the objects involved?

To my knowledge, no collisions are perfectly elastic although some collisions are well approximated by being elastic. Therefore, there will be some energy loss of the system to the environment, which may or may not be practical to consider based on the type of collision.

 

[alibongo]

Thanks for the replies.
My physics is so limited I'll show you what I'm trying to understand, rather than attempting to explain it.
Here's a clip with two fake crashes, which one is the most realistic in terms of physics?
The bottom one seems more realistic to me, with the deceleration, immediate ignition of the fuel, and breakage. I've never studied physics though, hence posting the question on here.

 

[xxChrisxx]

The first one seems slightly more likely, the fuel wouldn't explode as soon as the plane touched the building. What you don't see in the first one is that once it peirces the outer wall the plane is ripped to shreds, the engines and fuel tanks are ripped open inside the building. If you noticed what happened on 911 you'll see that the 'entry wound' to the building is actually very small. In fact I think the top video is real as it matches the sim exactly.

There was a decent combined simulation (plane and building) on youtube showing exactly what happened to the WTC and why it collapsed, it combines FEA and fire simulation into a single animation. Search for 911 simulation, its the top one.

 

[Danger]

The first one seems more realistic, except that I didn't see any damage to the building upon impact. It looks as if the plane is 'phasing' through the wall rather than punching a hole in it. (That might just be a problem with my eyesight, but I watched it 4 times.)
As Chris pointed out, there's no way in the world that poking a plane on the nose could rupture mid-ship fuel tanks, let alone spark them off.

 

[alibongo]

Danger, there's no problem with your eyesight, there is simply no damage to the wall.
I'm more interested in the deceleration though, shouldn't 500,000 tons of steel slow down the plane? At what point should the deceleration present itself?
(I'm not into conspiracy theories, I can't stand the 911 truth movement)
But this footage bothers me.
It's not just the Spell footage that shows no damage to the wall, no deceleration, and no breakage of the plane. Here are 3 more,

 

[xxChrisxx]

Thats the point its not 500,000 tomns of steel. The way the WTC is constructed its many many steel beams, that all together are incredibly strong but the plane only had to fly through a few of them. Take a look at that simulation I told you about, give me a sec and i'll link it.

The plane had the energy to smash through the first few, and planes are bloody stiff so its not surprising that it didnt break up instantly. As it flew through the structural beams inside it slowed down, that ripped the engines off and that's what caused the fire.

Imagine a razor hole (edit: slit) in a piece of paper, from far enough away you can't see it.

Edit and you can tell it had to fly into the building as the explosion came from the inside and blew out the side wall.

 

[alibongo]

Thanks Chris, but I'm curious about the transfer of energy. Shouldn't the plane slow down as it hits?
I didn't think planes were bloody stiff, I thought they were made with aluminium.
Shouldn't the plane have bucked immediately? Like Dangers comparison with the mosquito and the train.
Deceleration doesn't seem to be present in the vids I posted. If anyone can explain this please reply...

 

[xxChrisxx]

It does slow down as it hits, just not enough to notice on those videos. you can tell it slows down as it moves through the building as it takes longer for the debris to come out of the other side that it would take a plane to fly the distance.

Planes are made of aluminium, but its the construction (geomoetry) that makes them stiff. The plane deforms as it hits the building, but you've got the remember relaive to the width of the building the walls are paper thin and don't provide a large stopping force. Planes have to be incredibly stiff for saftey reasons, you can make planes much lighter than they are now but for crash saftey they are designed to stay together and the engine to snap off. (Which you will see in the simulation they did as it was into the biulding).

The reason why you can't see in it the vids is because they arent shooting fast enough and are too far away to see what's going on.

 

[buffordboy23]

I agree with xxChrisxx. When the initial impact occurs, you can think of the plane as 100,000-150,000 kilogram wrecking ball moving at a pretty decent speed; that's an incredible amount of momentum. So, any wall, even one with numerous reinforced steel beams, is not going to initially decelerate the plane that much to notice.

 

[Danger]

To take it to extremes, think of the 'straw in a hurricane' scenario. A regular stalk of hay or grass, traveling at a couple of hundred mph, can penetrate the bark of a tree. The aforementioned bullet is an unfortunate example as well.

 

[hookes law]

To take it to extremes, think of the 'straw in a hurricane' scenario. A regular stalk of hay or grass, traveling at a couple of hundred mph, can penetrate the bark of a tree. The aforementioned bullet is an unfortunate example as well.

Ah, but that's an urban myth!
The high forces of a tornado bend and twist telegraph poles and trees alike.
Near to the ground, a tornado can contain millions of flying pieces of grass, straw, twigs and things - it is simply one of these getting lodged into a bent tree or pole, the tornado passes, the tree bends back, and voila! A magic straw!

 

[Danger]

Ah, but that's an urban myth!

Not so. I don't dispute the rest of your argument, but the straw thing is real. You can demonstrate it yourself on a small scale by stabbing a potato very hard with a paper drinking straw. A fellow Canuck, whose name I can't recall, runs a lab that tests all manner of such things for the purpose of storm analysis. You wouldn't believe the amount of damage a 2x4 can cause when it comes out of the cannon at 300 kph.

 

[hookes law]

A fellow Canuck, whose name I can't recall, runs a lab that tests all manner of such things for the purpose of storm analysis. You wouldn't believe the amount of damage a 2x4 can cause when it comes out of the cannon at 300 kph.

Haha! Excellent!

Although having seen all the recent plane crashes on the news recently, I doubt a plane should have acted like that in the twin towers. Surely the tail section wouldn't have had the remaining force to cut through and leave a tail-shaped hole above where the rest of the plane has gone in? Not sure about the wing-tips. My gut tells me the same thing about them as well.

I would have expected a less plane-looking hole, if you see what I mean lol

 

[xxChrisxx]

Why wouldn't it? At that speed the tail would be like a knife going through the side of the building. After the initial impact the tail is torn off, the building is also structurally weakened by the front of the plane going in.

 

[hookes law]

Why wouldn't it? At that speed the tail would be like a knife going through the side of the building. After the initial impact the tail is torn off, the building is also structurally weakened by the front of the plane going in.

You know, I had a look at some of the videos, and the tail isn't torn off at all.
And it would have to be like a thinner, hollow knife going through a bigger, denser knife - I would expect to see it rotate downwards, or disintegrate against the wall.
There's something awry with some of these 911 videos. Hope no-one's making money from these...

 

[dE_logics]

When a moving object collides with a static object, is there always deceleration present at impact?

Yes.

Even if it collides with another moving object in the same direction there's always a retardation.

If the moving object is designed to pass through something else (a bullet or missile) I assume there is very little deceleration if any.

In the above explanation I did not assume this

Retardation is cause to third law, in this case, the third law didn't hold but it did apply, that means loss in momentum.

You know if a 7.62 mm bullet passes though Kevlar, it WILL slow down

Is there any way a car for example could pass through a brick wall without any deceleration, or breakage?

Then wall needs to be thin and there's NO WAY you can do that unless you're on a like......6000NM torque engine (i.e the engine needs to be running).


That too will be very less I think.

The answer lies not in the speed, or even the BHP but in the torque.

Since at higher speeds (cause of the lower gear ratio) the torque reduces by a huge amount, it should be preformed at a very low pace.

 

[Danger]

One thing that people seem to overlook when critiquing reconstructions is that we all saw the original footage just minutes after the event. There was no time for editing, let alone CGI enhancement. Not to mention how many hundreds or even thousands of eye-witnesses. It's pretty difficult to fake such a public event.

 

[xxChrisxx]

You know, I had a look at some of the videos, and the tail isn't torn off at all.
And it would have to be like a thinner, hollow knife going through a bigger, denser knife - I would expect to see it rotate downwards, or disintegrate against the wall.
There's something awry with some of these 911 videos. Hope no-one's making money from these...

You arent taking into account the geometry of the situation. The tail may be hollow, but it went in for all intents and purposed at a right angle to the wall. This means the tail would have been the most resistant to bending it could have been and the wall the least.

The entire plance was mashed in the middle of the building so it may or may not have come off, i'll have to re look at the videos.

EDIT: If you look at the simulation the tail is ripped off just after its entry to the building.

 

[hookes law]

One thing that people seem to overlook when critiquing reconstructions is that we all saw the original footage just minutes after the event. There was no time for editing, let alone CGI enhancement. Not to mention how many hundreds or even thousands of eye-witnesses. It's pretty difficult to fake such a public event.

Actually that's not true - the actual impact shots started appearing around midnight and then over the next few days, and the witness' statements varied from small plane to large plane to missile and even nothing...
But its just the impact videos that are interesting me at the moment.

 

[hookes law]

You arent taking into account the geometry of the situation. The tail may be hollow, but it went in for all intents and purposed at a right angle to the wall. This means the tail would have been the most resistant to bending it could have been and the wall the least.

The entire plance was mashed in the middle of the building so it may or may not have come off, i'll have to re look at the videos.

EDIT: If you look at the simulation the tail is ripped off just after its entry to the building.

You're quite right about the geometry, but I don't trust the simulations. The tail section in the impact videos stays straight and true throughout. I can't see any deceleration, the plane passes through the building in the same amount of frames it takes to travel its own length when in the air...

 

[Danger]

Actually that's not true - the actual impact shots started appearing around midnight

I admit that I might be mistaken on that issue. When I woke up at 7:30 in the morning (MST), it was on the news. My mother talked about it, as she had seen it at least half an hour prior. I still dispute, however, that there was time for CGI faking.

 

[xxChrisxx]

You're quite right about the geometry, but I don't trust the simulations. The tail section in the impact videos stays straight and true throughout. I can't see any deceleration, the plane passes through the building in the same amount of frames it takes to travel its own length when in the air...

Of course simulations with this magnitude of unknowns is inherently inaccurate, the general deformation of the plane is more or less correct. The simulation seems to match (roughly) time wise with the length of the impact and explosion, but without knowing the assumptions and the input data for the FEA its impossible to say.

And I don't understand, what's all this talk of CGI?

 

[Danger]

whats all this talk of CGI?

Computer Generated Images. Think of movies like 'Toy Story' or 'Spider-Man'.

 

[alibongo]

It does slow down as it hits, just not enough to notice on those videos. you can tell it slows down as it moves through the building as it takes longer for the debris to come out of the other side that it would take a plane to fly the distance.

The plane deforms as it hits the building, but you've got the remember relaive to the width of the building the walls are paper thin and don't provide a large stopping force.

The reason why you can't see in it the vids is because they arent shooting fast enough and are too far away to see what's going on.

You can tell it slows down because the debris takes longer to travel out the other side of the buiding?! Shouldn't you be able to see it slowing down as it impacts?
I ask again, at what point should the deceleration present itself?
Where in the videos I posted does the plane deform? The plane doesn't deform, it remains intact as it moves through the steel.
Are you saying the camera's frame rate isn't fast enough to capture the deceleration? You can see the plane fly into the tower the same speed it takes to fly flies through thin air. I'm not sure the frame rate is relevant.

 

[alibongo]

I agree with xxChrisxx. When the initial impact occurs, you can think of the plane as 100,000-150,000 kilogram wrecking ball moving at a pretty decent speed; that's an incredible amount of momentum. So, any wall, even one with numerous reinforced steel beams, is not going to initially decelerate the plane that much to notice.

A wrecking ball is dense and heavy, it's designed to smash through walls.

 

[alibongo]

One thing that people seem to overlook when critiquing reconstructions is that we all saw the original footage just minutes after the event. There was no time for editing, let alone CGI enhancement. Not to mention how many hundreds or even thousands of eye-witnesses. It's pretty difficult to fake such a public event.

Please, no talk of CGI enhancement. Why are you mentioning hundreds or thousands of eye witnesses? Eyewitnesses to what? Something probably did hit the towers, but the videos I posted look very odd.
What about the fuel ignition, I thought some fuel is carried in the wings. Shouldn't the fuel have ignited as soon as the wings broke up? Why can't we see the wings breaking in the crash vids I posted?

 

[xxChrisxx]

Computer Generated Images. Think of movies like 'Toy Story' or 'Spider-Man'.

I know what it is... :P I just don't see how you can 'fake' a ****ing enormous plane flying overhead. Especially the second hit, in which you can clearly see a huge plane.

If it walks like a duck and quacks like a duck...

What about the fuel ignition, I thought some fuel is carried in the wings. Shouldn't the fuel have ignited as soon as the wings broke up? Why can't we see the wings breaking in the crash vids I posted?

The fuel is carried by the wing, but nothing happens instantly in real life. And you can't see the wings breaking as the plane is in the building by the time they have come off. You've got to remember you can't see anything becasue the entire event (impact to explsion) takes place in about half a second. In shaky real time video you can't see squat.

The wings would hve taken a finite amount of time to break up, the fuel is is saftey cells to prevent a fire during a crash event, there would have required time to spread the fuel to an ignition source. Then it takes a finite amount of time for the combustion to begin.

EDIT: Aww I missed 100 posts :(

 

[alibongo]

There seems to be some confusion about the question I asked.
This thread is not about eye-witnesses, or how the plane could have been faked.

Can you post about the crash physics and show me where the plane appears to deform Chris?

Or make a new thread for discussing CGI, eyewitnesses, and how it could have been faked. I am also interested in these issues, but this thread is about the crash shown on the alleged footage I posted.

 

[xxChrisxx]

Its very difficult to show you where it deforms in a crash like this, becuase I don't keep a slew of plane crash physics videos to hand.

All I can do is talk you through the impact events, and the simulation vid seems to be vlid enough to talk through this.:

Nose hits and deforms inwards,
The fuselage is the stiffest is can possibly be being rammed through a wall at 90 degrees. So it shouldn't compress that much due to the impact.
The wings tear throug the outer wall of the building (releasing fuel) but are ripped off by the increased stress by going through a flooe and then finished off by the main supporting columns in the centre.
As this is happening the nose pitches down and the fueslage splits due to bending.
The tail remains intact throug the outer shell, but deflects to the side and comes off very soon after entering the building.

As the smaller sparts are moving through towards the central colums, the whole plane is torn to bits (there is no more accurate way of putting it than this).
About the same time as this the fuel hits a spark and the fuel ignites.

You won't see any of this in a video because a) it happens too quickly to see b) you arent superman and can't see through buildings.

 

[buffordboy23]

A wrecking ball is dense and heavy, it's designed to smash through walls.

Yes, this is obviously true, but your missing the point of why I made that remark. My point for making that comparison was to help you get an intuitive sense of why there is not a large deceleration of the plane upon impact. I suppose that there are better comparisons, but that's what came to my mind first.

 

[alibongo]

Yes, this is obviously true, but your missing the point of why I made that remark. My point for making that comparison was to help you get an intuitive sense of why there is not a large deceleration of the plane upon impact. I suppose that there are better comparisons, but that's what came to my mind first.

I am trying to understand, but your comparison makes me think if I drove my car into a wall at high speed, it might go through the wall without any deceleration.
Here's another clip.

 

[alibongo]

Its very difficult to show you where it deforms in a crash like this, becuase I don't keep a slew of plane crash physics videos to hand.

Fortunately I have a slew of plane crash videos. The one I just posted has much better resolution than the others but I still can't see any deforming.

 

[xxChrisxx]

You wont, not from any of these videos taken on the day. They are all too far away and can't be slowed down enough to show anything. All it'll show in a plane dissapearing into the side of the building. No matter how many you come up with you'll not see what you expect to.

To see the deformation you're talking about, you'd need a high speed camera positioned above the plane and to the side of the plane as it enters the building. This would also show that the plane decelerates. And by high speed i mean its shooting at least 1000FPS.

 

[hookes law]

Alibongo,
Do you have a video of the impact that is in slo-mo, and perhaps stabilised?

 

[alibongo]

Alibongo,
Do you have a video of the impact that is in slo-mo, and perhaps stabilised?

I'll look for one, that one I posted today is the best quality I've found so far.

 

[xxChrisxx]

you still won't see anything though, 25 fps is simply not a fast enough sample rate to capture what you want to see.

however we'll work with what we have and go from there, let us know if you find one.

 

[hookes law]

I've found it.
In the simulation, the tail drops down to fit into the hole, but I've tried slowing down one of the videos (the close-up one) and this simply doesn't happen in the video. There is enough frames of the tail going into show that (on the video at least) the tail does not drop down in such a way.

 

[xxChrisxx]

The tail drops? No it doesn't, the entire back end drops as the floor below gives way.
Look at the very first video he posted, the top vid is from the actual crash. You can see the back end drop in the same way.

The other two vids are of the second crash, and the simulation is of the first.

The physics of the second crash are slightly different as the flight is more level and the contact isn't perpendicular to the wall.

 

[hookes law]

The other two vids are of the second crash, and the simulation is of the first.

Ah, I see.
Maybe we should stop trying to compare details of the videos of the second crash to a simulation of the first.

 

[hookes law]

The physics of the second crash are slightly different as the flight is more level and the contact isn't perpendicular to the wall.

It doesn't look perpendicular to me either. Not exactly anyway. Should we see some rotation in that case, or is the wall, in effect, simply giving way with no measurable resistance?

 

[xxChrisxx]

i'm really not sure to be honest, i'd assume some rotational motion but not enough to see on the videos. i could be wrong though.

 

[hookes law]

i'm really not sure to be honest, i'd assume some rotational motion but not enough to see on the videos. i could be wrong though.

Yeah, I would assume some rotational movement as well. I'll see if I can find a good copy of the video and put my old computer skills to work :)

 

[alibongo]

I'm tired of waiting for Hookes Law to make a clip !
Here's a clip with the argument about the frame rate.



Although the 911 plane hit footage only has 25-30 frames per second, you can still see there's no deceleration, and no breakage of the plane.
Most film only has 25-30 frames per second, yet other crashes, such as in F1 racing, you can still see deceleration present.
I understand why the frame rate is such an issue. The plane doesn't slow down as it meets with the tower wall.
When should the deceleration present itself?

 

[buffordboy23]

Although the 911 plane hit footage only has 25-30 frames per second, you can still see there's no deceleration, and no breakage of the plane.
Most film only has 25-30 frames per second, yet other crashes, such as in F1 racing, you can still see deceleration present.
I understand why the frame rate is such an issue. The plane doesn't slow down as it meets with the tower wall.
When should the deceleration present itself?

The frame rate is not fast enough then for you to see any deceleration. You need a faster frame rate.

Here's another scenario. Wet a paper towel and have your friend hold it perpendicular to the floor. You now punch your fist through it as fast as you can. While there is a deceleration upon your fist during the impact, you will never see it.

 

[alibongo]

The frame rate is not fast enough then for you to see any deceleration. You need a faster frame rate.

Here's another scenario. Wet a paper towel and have your friend hold it perpendicular to the floor. You now punch your fist through it as fast as you can. While there is a deceleration upon your fist during the impact, you will never see it.

I punched the paper towel, I didn't notice any deceleration, but I did notice the towel break at the moment I punched it. The plane doesn't break at all as it enters the tower wall, it just glides in, shouldn't parts of the plane have broken off, especially the tail or wings? Also there's a delay of ignition, the fuel in the wings should have ignited the moment it hit, not a second or so after.

 

[xxChrisxx]

Jesus christ...

The deceleration is VERY SMALL. YOU CANNOT SEE IT FROM A POXY VID LIKE THAT. END OF STORY. Yet the laws of motion are not avoided, it hits the building therefore it must slow down by some amount.

Your F1 crash analogy is fail, you see deceleration owing to the fact that they have brakes on and are ususally traveling though a gravel trap. A typical big F1 crash takes place at about 150mph at point of impact. A typical crash may be upwards of 80g deceleration (this is hitting solid armco) you get an event time of about 2 frames. or about 0.08 seconds. for the main impact event. The cars wegith a shade over 600kg.

A thin wall of the WTC will in no way provide that amount of deceleration when something traveling around half the speed of sound and weighing probably 80 - 100 tons. The momentums and kinetic energies involved are not even in the same league.

I really with people would stop arguing there useles case without providing any scientific back up for thie rather stupid statements.

Your stupid points were as follows:

The fuel in the wings should have ignited the moment it hit?
Explain why you think for some strange reasion this should happen? (real crashes are nothing like the movies)Why should bits have boken off the plane as it hit?

The plane's wings is a hell of a lot stiffer than the outer skin of the WTC. So you would expect them to snap off when it met the structural beams inside the building. which is precisely what we see.

you arguements are utter utter crap. we've given you all scienficic reasons for why they are. stop persisting with them. or try to back them up with some evidence, or at least some measure of reasoning.

'because i think so' doesn't count.

 

[shure]

A thin wall of the WTC will in no way provide that amount of deceleration when something traveling around half the speed of sound and weighing probably 80 - 100 tons. The momentums and kinetic energies involved are not even in the same league.

Thin wall of the WTC?

Hmm quarter inch thick steel and 14 inchs in depth. Also 4 inch thick concrete poured over the steel truses on every floor, not to mention the steel core that the plane would come in contact with if it was possible to make it half way through.

Hello Chris, are you saying that if I was to hit an engine block with my fist fast enough, my fist could penetrate that engine block?

This thread was entertaining to read!

 

[xxChrisxx]

Although those figures about the wall being thick sound quite impressive, you are failing to appreciate the energies and forces involved. The fist example shows this to be ture.

Lets look at a comparison of energys for a moment.

A large .50 cal rifle round has on the order of tens of kilojoules. (approx 20kJ energy)

A 12lb cannonball fired from a napoleonic era cannon has hundereds of kJ (0.25MJ)

A plane traveling at that speed has approximately 1.2 Gigajoules of energy. This is the same energy as 0.4 tons of TNT. (although the method of destruction between high explosives and kinetic energy weapons is very different)

The reason for pointing out the above is to show that the idea of a plane acting like a slow moving object such as a fist striking the wall is incorrect. The plane should be viewed an an enormous bullet interms of its destuctive capabilites.

This thread is conerning the 1st imapct, which takes place roughly perpendicular to the outer wall of the building.

The main strength of the wall comes from the concrete, which as a material stands up to static compressive loading very well indeed. However concrete is hard and brittle, and perfoms poorly under dynamic loading and bending. Which is the loading that a plane impact would put on the wall.

The 'thin wall' description was used in context of the problem, when firing a bullet at a sheet of metal, when is that metal considered thick? Is a 1mm sheet thick? 3mm? half an inch...? As a general idea, the wall is considered thin when its critical length (in this case thickness) is << than the critial length of the striking object (in this case the length of the plane).

The plane itsself is basically an aluminium tube. Tubes are very stiff when loaded axially which is how the plane would be loaded during the impact.

During the impact event there is very little way to gauge the forces involved on the plane, we know that the impact event of the outer wall destroyed the wall but not the plane. This however does not mean the plane held up unsctathed. If you look at the simulations run, its shows that although the plane survives the inital impact well, the result of that loading deformesthat plane, and eventually lead to it being turned into shrapnel as it moves through the building.

It also indicates that the plane did indeed reach the central column, but by this time its was basically scrap metal. The main thing that confuses me about the impact is why the wings weren't ripped off, I would have expected this to happen due to the distributed load along the wing gausing a large moment at the wing/fuselage joint.Now let's take a look at your fist analogy. Relative damage to impact bodies depends on energy of impact, deceleration (which gives the forces) and the relative strength of the bodies in contact.

The simple answer is that if you could get your fist moving fast enough you could indeed damage an engine block. Probably not penetrate it due to your fist being made of flesh and bone, and the block being cast iron the difference in strength is very high. With a material fist sized but a strength closer to cast iron would likely penetrate the block

Lets look at the kind of speeds your 'fist o' steel' would have to be traveling at to theoterically penetrate the block. We need to take something of a similar size, so the 12lb cannon ball with do nicely. Assuming your hand is infinitely stiff and won't get turned to a bloody mess on imapct and that your fist weighs approx 1Kg.

To get a KE similar to that of the cannonball (which we know penetrates an engine block) you would need to punch the engine at 1.8 times the speed of sound.