Phyiscs Q&A

RenegadePhysicist · Post by **RenegadePhysicist** » 2005-08-22 01:23pm

Hello! I'm new to this board, but figured the best way to become familiar with your board is to dive right in and do what I do best: Physics.

I'll answer any questions, technical or otherwise, about physics theory. When I'm not going on my own knowledge but someone else's, you can trust that I'll provide references.

My greetings to you all, and I look forward to your questions!

Son of the Suns · Post by **Son of the Suns** » 2005-08-22 04:28pm

Errrr, if you don't mind me asking, how knowledgable are you? If you have a masters degree, then great, but if you are university student with a few physics classes under your belt, well, alot of us have at least that and then some. Usually if someone has a question they just post it in a thread and someone will answer it. You might want to wait for someone to actually post a question...

RenegadePhysicist · Post by **RenegadePhysicist** » 2005-08-22 06:05pm

Batchelor's degree. I'll be starting grad school at Stanford in the fall.

I don't know how this board works, so I'll take your word in regard to how people pose their questions.

Wicked Pilot · Post by **Wicked Pilot** » 2005-08-22 06:21pm

Actual no shit physics questions, usually relating to mechanics, come up every now and then, but not often. Perhaps when the new school year gets up into action you could provide homework answers in exchange for porn or something.

gizmojumpjet · Post by **gizmojumpjet** » 2005-08-22 06:42pm

Hey, I'll bite.

While it maddens me that I can't find a direct link in a timely manner, much attention is paid to the capabilities of Imperial and Republic era propulsion systems on our gracious host's site. One thing of note is how quickly SW ships can get from takeoff to open space via astronomical acceleration rates. Considering that planes at Mach 1+ can cause sonic booms, frowned upon by those upon the ground, I've wondered what sort of 'atmospheric' (or whatever the appropriate term is) effects an object the size of the Falcon would have taking off at the speed the Falcon exhibts sans any sort of technological apologies other than assuming that the Falcon-object maintains it's integrity.

Would the MF taking off from the surface of the Earth be destructive to the local natives, or just annoying?

SCRawl · Post by **SCRawl** » 2005-08-22 06:42pm

RenegadePhysicist wrote:Batchelor's degree. I'll be starting grad school at Stanford in the fall.

I don't know how this board works, so I'll take your word in regard to how people pose their questions.

In my observations, most of the thinking people here either know enough to answer their own physics questions, know how to look them up, or will post them as a new topic. There are already quite a few serious physics people here, but one more is always better.

The non-thinking people, in my humble opinion, can fuck right off.

(Incidentally, while I also hold a B.Sc in physics, I wouldn't offer myself as a decent source of information. I found out after about two years of physics that I'm really not all that good at it, but I'm bloody-minded enough that I got my degree anyways.)

kheegster · Post by **kheegster** » 2005-08-22 07:27pm

RenegadePhysicist wrote:Batchelor's degree. I'll be starting grad school at Stanford in the fall.

I don't know how this board works, so I'll take your word in regard to how people pose their questions.

Which branch of physics are you in? I'm contemplating applying for grad school in Stanford for either high energy astrophysics or cosmology.

The Grim Squeaker · Post by **The Grim Squeaker** » 2005-08-22 11:32pm

I'll bite more later this year when I have my final exams in Physics in school (Do you know anything about optics (angles of reflections etc...), or have a idiot proof explanation of the mechanics of quantum physics for my university course?).

For now I do have one question- what is the formula for calculating weapon yields based on destroying: A) a asteroid. B) a cube of rock.
Also how would you generate the yield of a weapon who's speed and mass are known fired in space. (v=.4c=120,000 miles/s=120,000,000 m/s.
m=50 tons=50,000 kg)

RenegadePhysicist · Post by **RenegadePhysicist** » 2005-08-23 12:52am

1) Given some level of hypersonic speed for the Falcon (That is, 5x the speed of sound or more), there would be a significant effect. Simple sonic booms can be pretty damaging to the eardrums, but in addition to this, the drag forces of the wind passing over the wing (remember, when wind passes over a non-flat object, its velocity will change as a result of the pressure difference) could conceivably blow over objects on the ground with quite a bit of force.

There's a more important implication:

Because the shock layer (distance between the shock wave from supersonic speed and the surface of the aircraft) is thin during hypersonic flow, the shock wave becomes highly curved (directly around the nose of the aircraft, in fact). I'll let Professor John D. Anderson from the University of Maryland take it away from here:

"The strong shock curavture induces large velocity gradients in the flow behind the shock in the nose region. These large velocity gradients are accompanied by strong thermodynamic changes in the flow. This region of strong gradients, called an entropy layer, extends downstream close to the body surface. Downstream of the nose [including behind the craft], the entropy layer interacts with the boundary layer growing along the surface; this interaction causes an increase in aerodynmaic heating to the surface, above and beyond the entropy layer."

In short, hypersonic speed literally 'heats things up.' Best example: a spacecraft returning to earth. Re-entry requires intense heat shielding for this purpose--not merely the friction with the air. Considering craft like the Falcon would be moving at much faster speeds than this, and considering temperature radiates outward, it'd be entirely possible for a craft moving at such speeds to create a searing path of destruction.

I'm really glad you asked this question. Aerodynamics is, to me, the coolest part of physics.

2) I'm probably going to focus on nuclear physics, simply because I'm also working in Aerospace Engineering (a second degree I picked up in undergrad). Once I'm done with my graduate studies, however far I manage to get without burning out, I want to work in the field of nuclear rocketry, as my father did back in the seventies. I had the opportunity to meet Dr. Harold Finger, the former head of the entire project, which was to date, the only joint endeavour of the Atomic Energy Commission and NASA. The government is restarting the program, and Dr. Finger got me interested in it.

If you want to get a 'true' physics experience in grad school, go theoretical. I'm signed up for a theoretical phys class on top of my current courseload just for fun. Back in undergrad I got to do a bit of research on string theory, and while I barely understood what I was doing, it was fun.

3) Strangely, my university never covered optics. I'm supposed to take a crash course 0-credit class on it this fall. As for the simple explanation of quantum mechanics, it goes based on a couple principles. One is the fundamental universal force group--gravity, electromagnetism, and the strong and weak nuclear forces. Electromagnetism vastly outmatches gravity on the quantum level, as do the nuclear forces. Without getting too technical--up close, the strong nuclear force is incredibly powerful, but quickly dissipates with range. Electromagnetism doesn't dissipate with range so much.

The big thing behind quantum mechanics is the Heisenburg Uncertainty Principle, which establishes that you can't know the exact position and momentum of an object (as I'm sure you well know). The equation for it, tying uncertainties in each to Planck's constant, can basically be used to determine an approxomate area where electrons COULD be (with certain areas more likely than others). That's where the probability sphere idea comes from. Quark theory is entirely mathematical and I don't think I could explain a word of it in english. Don't worry though, everyone will be equally clueless. I think I still am!

I doubt this is sufficient explanation, but the math part of it is a hundred times worse. I'm not great at putting quantum in simple terms, sadly. Granted, it doesn't HAVE simple terms.

4) In terms of weapon yield--assuming no explosion, you'd just want to use the relativistic momentum equation (different from m*v). It's something like m*v*lambda, but I think I remember momentum and energy being different from mass in terms of modification from relativistic velocity. This would be for some sort of mass-driver typed weapon. Assuming an explosion, you'd have to determine what percentage (if any) of the mass would be converted into energy. The old E=mc^2 relativity equation could be used here, to determine energy change as a result of the near-light velocity. I don't remember the exact equation but it's something to the tune of Erest*(lambda). You could use this to look at, at least, the mathematical suggestion of an improvement in total energy available for use by the weapon.

I'm no weapons expert, and this could all be total BS, but the fact that the velocity is relativistic will have some effect, whether there's a mass conversion to energy (as in the case of nuclear weapons) or just the basic bullet collision.

Post by **Darth Wong** » 2005-08-23 12:55am

Judging from your discussion of aerodynamics, I'd hazard a guess you didn't notice the part of his question where he said that the high-velocity object in question is in space.

RenegadePhysicist · Post by **RenegadePhysicist** » 2005-08-23 01:05am

His question was about atmospheric takeoff and the effect of supremely high speeds on surface life.

I treated it from the perspective of an object taking off at approxomately a 55 degree angle from the surface (the approxomate ideal angle of attack).

When did he say it was in space?

The Grim Squeaker · Post by **The Grim Squeaker** » 2005-08-23 01:10am

I'm no weapons expert, and this could all be total BS, but the fact that the velocity is relativistic will have some effect, whether there's a mass conversion to energy (as in the case of nuclear weapons) or just the basic bullet collision.

In the case of it being a physical projectile slammed ito an object pulverizing the projectile- what formula would I use to understand the measurement of the transfered energy and how would I convert it into kile/mega/giga tons.
(The object in question is a Super MAC from the Halo novelization- I want to learn how to find it's per shot yield and the underlying formula for future reference)

RenegadePhysicist · Post by **RenegadePhysicist** » 2005-08-23 01:17am

Well, first assume the collision is completely inelastic (as most real collisions are, and one in this situation probably would be).

Your basic conservation of momentum equation is:

m1v1+m2v2=(m1+m2)*v3

v3= (m1v1+m2v2)/(m1+m2)

In the case of a simple collision, there isn't any energy transfer--energy is the integral of force over distance. The only force exerted at any spatial point (within the projectile-target system) is that of whatever fired the projectile--after that, there is no actual force exerted on any object. Between the projectile and target, there is only one point at which there is energy transfer, and it is determined in a really simple manner.

Kinetic energy of the target = 1/2 * m*v1^2.

The target's new kinetic energy is 1/2 * m*v3^2 (remember not to count the added mass, as that'd defeat the purpose).

The target's energy should rise about as much as the projectile's fell.

Energy transfer = 1/2*m*(v3-v1)^2.

RenegadePhysicist · Post by **RenegadePhysicist** » 2005-08-23 01:33am

As a note, while you'd have to make the relativistic conversion, at .4c, lambda is still barely above 1: 1/(1-v^2/c^2)^1/2 = 1/(1-.16)^1/2 = 1/(.84)^1/2 = 1/.93. This is more than 1, but not much more. Leave a 20% error bound or so, and your answer will be in Joules.

Dennis Toy · Post by **Dennis Toy** » 2005-08-23 02:00am

are you versed in Quantum Physics? Are you familiar with the quantum computer. I got a few Q and A's about

1. How would data copying using entanglement be achieved. Lasers/Gas?

RenegadePhysicist · Post by **RenegadePhysicist** » 2005-08-23 07:39am

Quantum mech, I understand--but I know next to nothing about the quantum computer, sorry. That's a concept WAY over my head! But I wouldn't be surprised if someone else did know about it.

Il Saggiatore · Post by **Il Saggiatore** » 2005-08-23 07:44am

Dennis Toy wrote:are you versed in Quantum Physics? Are you familiar with the quantum computer. I got a few Q and A's about

1. How would data copying using entanglement be achieved. Lasers/Gas?

Maybe this can help: www.qubit.org .

Thinkmarble · Post by **Thinkmarble** » 2005-08-23 08:23am

RenegadePhysicist wrote: 3) Strangely, my university never covered optics. I'm supposed to take a crash course 0-credit class on it this fall. As for the simple explanation of quantum mechanics, it goes based on a couple principles. One is the fundamental universal force group--gravity, electromagnetism, and the strong and weak nuclear forces. Electromagnetism vastly outmatches gravity on the quantum level, as do the nuclear forces.

For a basic explanation of quantum mechanics I would not start to list fundamental interaction, but I would rather list the underlying principles.
Basically you treat physical states as vectors in a hilbert space while treating measurement as hermitian operators.
Possible values of an measurements are eigenvalues of the operator, expected mean average of an measurement is the product of an physical state with the operator applied to the physical state.
The system of eigenvectors to an hermitian operator is an complete orthonormal base.
The eigenvalues of an operator are possible measurement values, to get the probability to measure a specific eigenvalue you take the modulus square of the product of the specific eigenvector with the state of the system (the coefficients of the representation in the eigenbase are the probability amplitudes to finde the system in the corresponding eigenstate).
And then there is the dreaded collapse postulate.
A restatement is in order:
Physical states are vectors <psi| in a hilbert space
(most of the time you use normalized states, but not always)
Eigenvalues of an operator are possible measurement values
O*|o_i>=o_i|o_i>
To get the probability to measure an value project the state on the eigentstate |<psi|o_i>|^2=p(o_i).
If we measure a value and then make a second measurement we want to get the same value.
So if
p_1(o_i)=|<psi|o_i>|^2
and
p_2(o_i) of the second measurement shall be one
then it follows that the state of the system after the first measurement has to be in the eigenstate <o_i|.

p_2(o_i)=|<o_i|o_i>|^2=1

That is called collaps of the wave function as the wave function goes from <psi| to <o_i| in zero time.

Collapse postulate is part of the standart/minimal interpretation of qunatum mechanics.
Some interpretation promises to do away with it (eg. Many worlds interpretation for example), and decoherence theory attempts to explain the collapse (talking about time evolution of density matrices representing a quantum mechanical system coupled with an termal bath), also I do not know enough of about it to render a judgement what I have seen looks promising.

Without getting too technical--up close, the strong nuclear force is incredibly powerful, but quickly dissipates with range.

If you model the color interaction outside of chromodynamics you do so by an potential with a term growing linear with the distance.
Fun part is that you get energy density fast were pair creation of particles with color charge becomes possible.
So you drag two particles apart and after a short distance particles are created which bound with the original particles and you get composite particles which are "white".
So the strenght of the interaction does not decrease, and that is exactly the reason why it is short ranged.
(this is also known as quark confinement)

Electromagnetism doesn't dissipate with range so much.

The big thing behind quantum mechanics is the Heisenburg Uncertainty Principle, which establishes that you can't know the exact position and momentum of an object (as I'm sure you well know).

The Heisenberg uncertainty principles is more general.
It relates the standart deviations of any two operators and there commutator.
If two operators have the same system of eigenstates, then it does not matter wich measurement is done first.
But if they do not commutate the order of application becomes important.
Take two operators A,B with the same eigenstate system.
A|es_i>=a_i|es_i>, B|es_i>=b_i|es_i>
We have an physical state |psi>
First measurement: We measure A
|psi> becomes |es> and we measure a
Second measurement:We measure B
|es> becomes |es> (as the system is already in an eigenstate of B the measurement does not changes the system)
and we measure b
Third measurement:We measure A again
|es> becomes |es> and we measure a again, surprise surprise the same value we had after the first measurement.
Things change if we have two operators which do not commutate with each other:
First measurement: We measure A
|psi> becomes |a> and we measure a
Second measurement:We measure B
|a> becomes |b> and we measure b
Second measurement:We measure A again
|b> becomes |a*> and we measure a*
a and a* are in general different values

Quark theory is entirely mathematical and I don't think I could explain a word of it in english.

You mean quantumchromodynamics ?
The difference between QFTs and QM is:
- system of fields instead of particles
In QM you have electrons described by wavefunctions, in quantum field theories you have electrons as quants of an underlying matterield
- interaction mediated by field quants
in QM interactions are treated as classical potentials, in QFTs they are treated as quantizied fields
- interactions are created by gauge groups

Don't worry though, everyone will be equally clueless. I think I still am!

If there is an interest I can write out how you get from your gauge group to your interaction for an simple example.
(Klein-Gordon Lagrangian and local U(1))

I doubt this is sufficient explanation, but the math part of it is a hundred times worse.

I've found the math for quantum mechanics/dynamics always to be easier then words or the math for fluid dynamics or GRT

I'm not great at putting quantum in simple terms, sadly. Granted, it doesn't HAVE simple terms.

4) In terms of weapon yield--assuming no explosion, you'd just want to use the relativistic momentum equation (different from m*v). It's something like m*v*lambda, but I think I remember momentum and energy being different from mass in terms of modification from relativistic velocity. This would be for some sort of mass-driver typed weapon. Assuming an explosion, you'd have to determine what percentage (if any) of the mass would be converted into energy. The old E=mc^2 relativity equation could be used here, to determine energy change as a result of the near-light velocity. I don't remember the exact equation but it's something to the tune of Erest*(lambda). You could use this to look at, at least, the mathematical suggestion of an improvement in total energy available for use by the weapon.

I'm no weapons expert, and this could all be total BS, but the fact that the velocity is relativistic will have some effect, whether there's a mass conversion to energy (as in the case of nuclear weapons) or just the basic bullet collision.

The relation you are looking for is:
E=m*c^2*gamma with gamme=sqrt(1-v^2/c^2)^-1

That is kinetic plus mass energy, if you are only interested in kinetic energy you have to substract mass energy

Dendrobius · Post by **Dendrobius** » 2005-08-23 09:49am

simply because I'm also working in Aerospace Engineering (a second degree I picked up in undergrad)

So I'm guessing you're a double degree person, you've actually got a B.Sc/B.Eng (Aero) [In Australian terms]?

I'm just curious because a standard B.Eng degree is a four year course, not exactly something you pick up along the way as an undergrad unless you're in a dedicated double degree program, so kind of wondering how that works overseas.

I'll let Professor John D. Anderson from the University of Maryland take it away from here

Ahh, Anderson, the author of what would be equivalent to the "Dummy's Guide to Aerodynamics and Design" textbooks. Invaluable for a guy who doesn't know what he's doing to get a good grounding or an undergrad cramming for his exam.

Finally, my proper question:

I treated it from the perspective of an object taking off at approxomately a 55 degree angle from the surface (the approxomate ideal angle of attack).

What is this "ideal angle of attack" that you're speaking of? I've never heard of it before. A 55 degree angle of attack is insanely high for anything generating lift, and you can't possibly be referring to a rocket since you're talking about angle of attack...

gizmojumpjet · Post by **gizmojumpjet** » 2005-08-23 09:59am

RenegadePhysicist wrote:His question was about atmospheric takeoff and the effect of supremely high speeds on surface life.

I treated it from the perspective of an object taking off at approxomately a 55 degree angle from the surface (the approxomate ideal angle of attack).

When did he say it was in space?

You got it right. I was curious regarding the implications of a SW speed transit from the surface into space and you confirmed my suspicions. Considering we never hear any sonic booms or see ionization trails of outbound starships around ports like Mos Eisley or the heavily trafficed Coruscant, I'm assuming that there must be some sort of system in place to negate the effects.

Another possibilty, though, is that all of the quick launches we see in the films were actually emergency or "war power" launches that did cause surface destruction, and that this unfortunate fact has been covered up by New Republic propagandists.

Magnetic · Post by **Magnetic** » 2005-08-23 11:42am

gizmojumpjet wrote:
RenegadePhysicist wrote:His question was about atmospheric takeoff and the effect of supremely high speeds on surface life.

I treated it from the perspective of an object taking off at approxomately a 55 degree angle from the surface (the approxomate ideal angle of attack).

When did he say it was in space?
You got it right. I was curious regarding the implications of a SW speed transit from the surface into space and you confirmed my suspicions. Considering we never hear any sonic booms or see ionization trails of outbound starships around ports like Mos Eisley or the heavily trafficed Coruscant, I'm assuming that there must be some sort of system in place to negate the effects.

Another possibilty, though, is that all of the quick launches we see in the films were actually emergency or "war power" launches that did cause surface destruction, and that this unfortunate fact has been covered up by New Republic propagandists.

I could be wrong, but for a ship to accelerate at high speeds, there would have to be some sort of dampener, anti-gravity generator, (or the like) on board or all of the non-bolted down items would be thrusted directionally in the oposite direction of the path of the ship. So, if these types of technologies keeps people and things in place during these manuevers, then perhaps they are a ship wide effect (including the outer hull), thus the drag force of the outside air around the vessel would not obtain supersonic speeds, thus there would be no sonic air disturbances.

I'm not a physicist, nor do I have a degree in physics. . . . . . . . . . but I DID stay in a Holiday Inn Express last night!

gizmojumpjet · Post by **gizmojumpjet** » 2005-08-23 12:47pm

Magnetic wrote:I could be wrong, but for a ship to accelerate at high speeds, there would have to be some sort of dampener, anti-gravity generator, (or the like) on board or all of the non-bolted down items would be thrusted directionally in the oposite direction of the path of the ship. So, if these types of technologies keeps people and things in place during these manuevers, then perhaps they are a ship wide effect (including the outer hull), thus the drag force of the outside air around the vessel would not obtain supersonic speeds, thus there would be no sonic air disturbances.

You're not wrong; certainly in SW and generally in SF spacecraft maneuver at velocities that would reduce humans to goo without some sort of compensation system.

Keeping the air around the vessel in place while it exits the atmosphere at astonishing speeds still doesn't solve the problem, since those in place atoms of air are still going to drag against the air not held in place by the compensation force, resulting in friction and it's attendant heat.

Obviously they do something, but to speculate on the specifics moves quickly into the realm of handwavium.

Son of the Suns · Post by **Son of the Suns** » 2005-08-23 08:18pm

Strangely, my university never covered optics.

You never covered optics??? What about waves and acustics?

RenegadePhysicist · Post by **RenegadePhysicist** » 2005-08-23 11:03pm

1) I went to UMD, where the entire focus of the four-year program in physics was preparation for grad school in the realm of theoretical physics. That's all UMD's grad school for physics does, really--their powerhouses are theoreticians like Dr. Jim Gates, who is one of the leading string theoreticians. We had a semester course on waves, but it almost entirely focused on the wave equation and such--optics were left out. In my opinion the course was not well-designed, and I didn't get a great deal out of it.

2) In terms of 'everything not bolted down flies all over the place'--in short, modern fighter aircraft really do just bolt everything down. As gizmo said, it's the realm of handwavium. :-p

3) I'd guess that they don't have aerospace engineers determining the risks of collateral damage from the entropy layer generated by hypersonic travel in the SW movies. That's just one of those times where we have to say "Ewoks did it."

4) Yep, Dendrobius, I completed a dedicated double degree program. I took quite a painful load over four years, usually taking 18+ credit hours. Not fun, but I'm glad I did it now, because I can study both areas. And yep, Anderson is the author of choice for explaining aerodynamics in simple terms. He's a pretty nice guy too; he taught my aeroelasticity class, and we got along well.

5) The 'ideal' angle of attack is the one where Lift/Drag reaches its maximum. Normally this angle is remarkably low, probably below 5 degrees. However, hypersonic flight is totally different, and its Lift/Drag ratio maxes at a much higher value. This is important for hypersonic flight, because the ratio also decreases much more quickly as velocity increases.

drachefly · Post by **drachefly** » 2005-08-29 05:12pm

The crux of the 'compensation' in this case is that the air does not impact against a firm object like a wing, but against some sort of force field. This would not cause the typical zero-velocity boundary condition one has at a surface. The extended field would make the velocity field come out much more smoothly. And that would reduce turbulence and the energy imparted to the atmosphere.

In short, it could very well be that a ship with inertia compensation produces a milder shock wave than would be expected using normal physics.

And RoguePhysicist, you're not exactly alone in the knowing-something-about-physics department. Stay within what you really know and you won't find yourself in an embarrassing situation.
::coughs, glances nervously from side to side::