# Mythbusters 7x06 "Exploding Bumper" - Discuss and Grade

Discussion in 'TV & Media' started by Hermiod, May 13, 2009.

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5. ### Poor (Myth Busted!)

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More Mythbustery:

2. ### LitmusDragonCommodoreCommodore

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The archer one sounds interesting. I would imagine that the kinetic energy of an arrow fired from a horse at speed would be greater than that fired by a stationary archer, but how much greater?

I'm sure the exploding bumper one is based on some urban legend I've never heard of before, will wait until the episode to give judgement.

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Nov 27, 2004
Hm, I don't see how the speed of the horse changes the speed of the arrow? If someone shoots a gun from a moving car the muzzle velocity still is the the same as when the gun is shot from a stationary point also, right? Or am I missing something?

4. ### LitmusDragonCommodoreCommodore

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I'm no physics expert, but- my thinking is that based on the principles of momentum, and the law that an object in motion tends to remain in motion, that the more force behind the arrow at the point of launch the greater the tendency for that object to remain in motion, therefore resulting in slightly more penetration of the arrow. The question is whether any of the horse's momentum would be transferred to the arrow through the bow mechanism. It seems intuitive to me that a spear thrown from horseback would be deadlier than one thrown from a standing position, but I am not so sure about an arrow fired from a bow. I think perhaps yes. Twice the penetration seems unlikely however.

edit:
Something else to back this up- think about how javelin throwers run forwards before throwing the javelin. This is presumably to transfer their forward momentum onto the throw so as to achieve greater distance. However this is a comparatively larger projectile being propelled at a significantly slower speed than an arrow from a bow, so the same principle may not apply at the scale on which arrows and bows operate.

Last edited: May 13, 2009

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Nov 27, 2004
Maybe.

But before knowing what the Mythbusters think about it, my best guess would be, that it's a myth that resulted from the fearsome reputation of the eastern cavalry-archers in the middle ages, which was primarily due to their enormous strategic and tactical advantages compared to other European armies of the time as far as I know, and not because of superior weaponry.

Joined:
Mar 15, 2001
In a vacuum, a projectile fired from a moving vehicle would go faster than one fired from a stationary vehicle, because the velocities would add together. E.g. if the projectile is fired at 20 m/s and the vehicle is moving at 10 m/s, then its velocity relative to a stationary target would be 30 m/s, so it would have 1.5 times the velocity and therefore 2.25 times the momentum.

However, if we're talking about a projectile fired through the air, then terminal velocity must be considered. If the arrow travels at terminal velocity when fired from a stationary bow, it wouldn't be any faster fired from a moving bow, and therefore wouldn't have any more momentum. So the key question is, is the velocity imparted to an arrow by its bow sufficiently below terminal velocity that the added speed of the horse would make a difference?

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Trekker4747
Hee Hee.

"twice the penetration."

:giggle:

Trekker, 12.

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@ Jamie tipping the grade-all forward.

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Exploding bumper: Oh, so that's how it's supposed to work. I didn't know there were shock-absorbing pistons in modern bumpers. It makes sense on the face of it that a pressurized tube of gas could potentially explode if heated. But as their first test showed, it's not something that just happens as a matter of course.

Not surprised that Adam couldn't start the fire until he went into the, uhh, cockpit? cabin? What do you call the inside of a car? Anyway, in that larger space, the gasoline vapor wouldn't have been as concentrated, so it would've been closer to the right ratio of gas fumes to air.

An interesting set of tests with the piston. It's reassuring that these car parts are designed so well that even the Mythbusters can't make them fail dangerously. And the guys showed they have a good grasp of physics -- Adam's idea about the bumper as a heat sink, Jamie putting the metal plates on to diffuse the flame.

It's an unusual situation for the Mythbusters: they get direct, firsthand confirmation that this event has happened, so the myth's confirmed, but they can't reproduce it or figure out exactly how it had to happen. That's kind of frustrating.

So of course they faked it out with gunpowder. And they call it busted, which I guess it is on the specifics of the myth as stated, that the bumper could fly 50 feet. But I'd call that just an exaggeration rather than a myth, since it's confirmed that a bumper can explode outward in a car fire, however improbable that may be.

Horseback arrows: Both the myths this week are above average for this season so far. I like it when they do things from history like this, and the physics is interesting. From the first test, it looked like the speed of the horse did increase the arrows' velocity and momentum, so as Kari said, that left the question of how momentum correlates to penetration. It should increase penetration, since force equals mass times acceleration. But by how much?

This seems to have been taped earlier than some of the other myths, since Kari doesn't look pregnant. Actually I thought she did when riding the horse, but back in the shop where she was carving the horse's head (and I'm wondering where the obligatory Godfather joke was), she was wearing a close-fitting top that showed no sign of pregnancy. And I doubt they would've done the first test a month or two after planning the second, although I suppose it's possible that portions of the testing were staged/recreated for the cameras.

The car/crossbow tests went well and provided some good data. I would've liked to know what the arrows' actual speed was so I could figure out if that 31% increase in velocity was accounted for by the 40 mph of the car (which would mean the arrows' speed from a static start was 129 mph), or if it was less than that and air resistance was diminishing the velocity increase. Anyway, this is the same as the previous myth: the underlying concept is confirmed, but the alleged magnitude of the effect is busted. Firing while in motion does increase the speed and penetration, but not to the extent believed/claimed.

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Exploding Bumper:

Not surprised by the result on this. It seemed like something that requires an extraordinary set of circumstances to have occur and that safety features would be designed in the piston to prevent it. They should've, however, gotten the same year/make/model of car that injuried the fire-fighter as it seemed that at the very least that car was susceptible to it. But even if they could duplicate the explosion, the bumper traversing 50 feet would be incredible.

Double Pentration Bow and Arrow.

Not surprised it didn't double the penetration or that it did increase the penetration.

Also, Kari in the warrior-chick outfit? HOT!

Oh.

And I know it's more common and typical to use Celsius in "scientific" settings, but the vast majority of the US TV audience has no concept of the scale. That is they likely don't know "how hot" 600-some C is. I know I don't know how hot 600C as I've no practical concept of that temperature scale. Telling us in Fahrenheit would've given the vast majoirty of their audience a better understanding of the temperatures we were dealing with.

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^Just multiply by 1.8 and add 32. 0.8 x 600 = 480, so it's 600 + 480 + 32 = 1112 F.

Besides, how much did we really need to know beyond "really really hot?"

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Yeah, yeah, I know how to convert it. (I did go to high school, afterall ) but it's "more work than I should have to do" to understand how hot things are.

"1112" tells me something. Like that that's about half as hot as lava, that it's about three times hotter than the ignition point of paper, etc.

"600 C" doesn't help at all to know how hot things are other than, "maybe six times hotter than the boiling point of water."

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Mar 15, 2001
^See, I don't get "more work than I should have to do." Watching TV, especially a show like this, shouldn't be about being a passive sponge and getting all the answers fed to you. It should stimulate the viewer to do some thinking and work things out for oneself.

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The numbers game seems disappointing to me. It didn't travel *50* feet, it wasn't *twice* the penetration. I mean, bumpers can explode, shooting arrows from horseback does increase penetration, and so on. Especially the bumper thing. Still interesting though.

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Trekker4747
Perhaps, but it'd still be easier for the average viewer to say the temperature in units they're familiar with. So they don't have to pull out a calculator or look up formulas to "understand" the temperature they're talking about.

Would it have really been that hard for the narrator to say something like, "The shock has reached a temperature of 600 degrees, or about 1100 degrees Fahrenheit."

It's just a quibble. Not a real big deal. I mean I understand the ratios enough to get an "idea" of the temps they're talking about but it's still a bit of a quibble. I'm an American and I like my Imperial system, dammit!"

(I'd also argue that Fahrenheit is a more precise system. )

Joined:
Mar 15, 2001
Well, technically yes, it's more precise in the sense that its individual units are smaller. But that's what decimal places are for. Either scale can be as precise as you want by adding enough decimals. And at least the 0 and 100 points in the Celsius scale are defined based on simple and non-arbitrary standards.

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Nov 27, 2004
And that would be a rather stupid argument.

But I understand your problem, I also have no idea what xx Fahrenheit is, or xx feet, without converting it in my head.

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Jan 31, 2007
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Hey, we had this one in the UK on Monday - are we ahead of the US now?

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There are 180 whole units between Freezing and Boiling points of water in F.

100 in Clesius.

F is "more precise."