Energy Dissipation Of A Sphere Passing Through a Medium

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Hancock the Superb
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Re: Energy Dissipation Of A Sphere Passing Through a Medium

Post by Hancock the Superb »

Simply that rotational kinetic energy recieved at the time of firing still needs to be dissappated before the projectile will truely be stopped (the previous solutions only account for translational kinetic energy). This rotational energy results in greater penetrating power, as evidence by the destruction of Fort Pulaski.
Hancock the Superb
Marching Thru Georgia
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Re: Energy Dissipation Of A Sphere Passing Through a Medium

Post by Marching Thru Georgia »

Please show how rotational energy adds to the forward momentum of a projectile. This is a phenomena I am not familiar with. Rotation stabilizes a projectile in flight. In other words, it increases it's accuracy through the law of conservation of angular momentum. This is why it's easy to stay upright on a moving bicycle and hard to do so on one that is stationary. Draw yourself a vector diagram to show which way the angular and forward momentum point.
I can make this march and I will make Georgia howl.
Hancock the Superb
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Re: Energy Dissipation Of A Sphere Passing Through a Medium

Post by Hancock the Superb »

MTG: You are damn good at twisting my words around or my style of communication must be more confusing than jibberish.

The rotational kinetic energy affects the friction, not the initial momentum. At this point, my limited knowledge of physics dissipates (pun intended), and I will proceed off of pure conjecture, based in logical reasoning.

The projectile possesses a certain amount of kinetic energy. This energy must be matched by an equal reduction in energy from friction. So we cannot only take away translational kinetic energy, for the object will still be rotating. Friction being the only acting force, the rotational energy must dissipate through friction as well.

Friction over a distance equals work and tangential force over a distance (as distance = THETA / radius) is also equal to work. Since the friction can be applied to any point on the object and have the same effect (Since the basic identity of friction is f=uF[sub]n[/sub]), I will argue that the work friction does reduces that total energy of the object, not just the translational energy.

This should make sense for anyone who tries to put a nail through tough container board. You may be able to push the nail in a little bit by just shoving it in, but you can push the nail through if you add swivel it back and forth. I ask: if you cannot push down on the nail any harder without twisting, why should it go through with twisting? I respond: you are adding energy, as torque and force can be mathmatically transformed into to work in this context (multiplying force by distance, etc). So the total energy has been increased, but not the downward force (for as MTG says, rotational energy does not affect velocity or forward acceleration). This energy is combated by the work of the friction force, but there is some energy to push the nail through the container board, unlike the first example, where the force you apply and the friction are equal (thus you are unable to push it in any more).

Of course, what I just wrote could be entirely bogus, but then explain to me why when I try to pull a nail out of wood with the tail of a hammer, if I cannot get it out by just pulling, the nail will come out when I pull and twist it simultaneously.
Last edited by Hancock the Superb on Thu Nov 29, 2012 5:41 am, edited 1 time in total.
Hancock the Superb
Marching Thru Georgia
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Re: Energy Dissipation Of A Sphere Passing Through a Medium

Post by Marching Thru Georgia »

Hancock The Superb wrote:
At this point, my limited knowledge of physics dissipates (pun intended), and I will proceed off of pure conjecture, based in logical reasoning.
And again, this is where you get yourself into trouble. As I said before, hand waving arguments are fine in fluff subjects like history or social studies, but in the physical sciences you have to do the calculation to justify your position. The rotational momentum is orthogonal to the translational momentum. I think part of your answer shows you know this. So the question arises, how can forces at right angles to each other, influence each other? The answer is they can't. This should have been one of the first things you learned in high school physics.

So now let's examine the nail and the wood puzzle. Since we know the twisting does not add any force to the nail being moved through the wood. What is going on? In the case you describe, there are two kinds of friction, static and kinetic. This you may very well not be aware of. Static friction must be overcome to get an object to begin moving. Once it does, kinetic friction must be overcome to keep an object moving. The two have different magnitudes. So in the case of the nail, the twisting simply overcomes the static friction. The pushing on the nail at the same time overcomes the kinetic friction.

Do you see how that is a different problem from the case of an already moving projectile?
I can make this march and I will make Georgia howl.
Hancock the Superb
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Re: Energy Dissipation Of A Sphere Passing Through a Medium

Post by Hancock the Superb »

You logic seems completely correct. Yes, I do seem to have forgotten to take static friction into account. Such are the problems of a high school senior who can only draw upon his current physics class to help him. Out of curiosity, may I further ask where the rotational energy goes?

As far as my hand waving arguments, my sentences are merely physics equations in sentance form. If you wrote each sentence in equation form, you would notice that the units line up correctly as well as the majority of the vectors (I am rarely willing to say I am 100% certain). My mathematical work was done. Static friction is one thing I forgot about. And what is the point of physics if you cannot make some argument from it? We don't have electron orbital models to needlessly fill miserable students minds with information, we have them because now we can make an argument about bonding patterns and such.

Finally, I award you full points ;) for keeping a non-condescending tone in your second paragraph. It might be interesting to see what would happen if you could hold the disparaging comments in the first paragraph as well as previous posts. I think most people would agree that I have been as polite as possible to everyone on the forum, and I would hope that the same would be returned. I suggest this helpful advice because you have helpfully corrected my mistake this time (last time I think you misunderstood my study).

On the bright side, next year I will have little free time (as I will be running and studying nuclear engineering), so you will not have to suffer through to have interesting physics debates with me.
Last edited by Hancock the Superb on Thu Nov 29, 2012 6:56 am, edited 1 time in total.
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