User talk:Wolfkeeper

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Translation with no acceleration

On Talk:Centrifugal force you wrote:

"Just do a coordination translation to everything in the eccentric frame so that the origin now rests on the rotation axis. Since a translation doesn't change any accelerations you have now created the same situation as the article covers."...

A translation may change acceleration. In this case, it doesn't because the position of the new origin, as seen from the old frame, (i.e. the translation vector) is fixed, i.e. time-invariant. In the case of a particle moving along an S-shaped path, or any non-circular and non-rectilinear path (this is the situation described in the article, isn't it?), the translation vector is time-variant, and it has non-null second derivative.

Believe me, there's no way to give an answer to my question using just mathematics. For instance, in the article about fictitious forces, in the example about orbiting (but not rotating) frames, you have a situation in which it becomes clear that the fictitious force that we conventionally call "centrifugal" actually appears in the orbiting frame as a uniform force field (every point in the orbiting frame is acted upon by the same centrifugal force), rather than radial! It appears clear that calling this force centrifugal is just an arbitrary decision. You might very well call it "universal force" or whatever. This is yet another example in which what we call centrifugal force does not depend on position.

Math just tells us we have a fictitious force. We all know how to compute this force, but formulas give us a number, they don't give us a name for that number. The problem is just how you decide to call it (and of course, how the scientific community agreed to call it by convention).

Now, according to you, PeR, and Brews, the world agreed to call it centrifugal; I am not 100% sure that all authors agree about that, but I actually don't care, as long as you all agree! And I am glad we can simplify our task by adopting a conventional answer on which we all agree.

Does all of this make sense to you? Paolo.dL (talk) 20:25, 15 July 2008 (UTC)

Not completely....- (User) WolfKeeper (Talk) 21:46, 15 July 2008 (UTC)

After reading my recent comment about the car along S-shaped trajectory on Talk:centrifugal force, did you change your mind about the non-null second derivative of the translation vector? (please answer in this page) Paolo.dL (talk) 10:55, 16 July 2008 (UTC)

I think you just need to include Euler, coriolis and frame acceleration forces themselves as the center of the frame rotation moves and frame rotation speed changes. But it's all very messy and I don't think it's worth worrying about too much.- (User) WolfKeeper (Talk) 13:27, 16 July 2008 (UTC)

Sentence about frames accelerating relative to each other

From previous section: ... In the case of a particle moving along an S-shaped path, or any non-circular and non-rectilinear path (this is the situation described in the article, isn't it?), the translation vector is time-variant, and it has non-null second derivative. ... Paolo.dL (talk) 20:25, 15 July 2008 (UTC)

From User talk:Paolo.dL: No. The translation vector is due to the reference frame origin being offset from the rotation axis for the frame. Any particle who moves within the reference frame has nothing whatsoever to do with it; the *translation* is a fixed coordinate mapping for the entire frame.- (User) WolfKeeper (Talk) 21:46, 15 July 2008 (UTC)

After reading my recent comment about the car along S-shaped trajectory on Talk:centrifugal force, did you change your mind about this? Do you agree that the two non-inertial frames accelerate relative to each other? I.e., do you agree that the translation vector from each other is time-variant, and its 2nd derivative is not null? (I cannot understand your point: of course, for any instant of time, the translation is valid for the entire frame, but this has nothing to do with the 2nd derivative of the translation vector). Paolo.dL (talk) 15:11, 16 July 2008 (UTC)

I don't find the question, if I understand it, particularly interesting. The driver can be considered to be accelerating between rotating reference frames that are centered on his instantaneous center of curvature. In each of those reference frames he is subject to centrifugal and coriolis force.

If you try to analyse it from the frame of reference where the driver is the origin, you have the additional complication that the axis of frame rotation doesn't coincide with the origin. The frame is then a fully accelerated one.- (User) WolfKeeper (Talk) 15:45, 16 July 2008 (UTC)

I am just trying to understand your comment. The sentence we are discussing actually explains the reason why the S-shaped motion (the third example used in Talk:centrifugal force) is different and more interesting than a rotation about a fixed axis (the previous two examples in Talk:centrifugal force). The interesting difference is that the two non-inertial frames (attached to driver, and attached to center of curvature) are not equivalent when the car moves along and S-shaped trajectory, because they accelerate with respect to each other (although they rotate at the same rate). On the contrary, in my previous example (car running along circular path = "orbiting and rotating" body), the two non-inertial frames were almost perfectly equivalent (i.e. neither moving nor accelerating relative to each other). Your "no" seemed to deny this difference. This difference is important because it is the reason why you are almost forced to choose the LCS system (i.e. the system fixed to the car driver, the most natural system for him/her). Did your "no" become a "yes"? If it is still a "no", I warn you that I still cannot understand it, and this might hinder our discussion. Paolo.dL (talk) 16:12, 16 July 2008 (UTC)

Orbit Unclarity

I read the article, Orbit, and I don't understand the cannonball scenario. Don't projectiles, such as the cannonball, follow parabolic paths? I suppose the eccentricity is an absolute value thing in this case where the orbit goes from parabolic to elliptical to circular to elliptical, then parabolic, again, and finally to hyperbolic. According to geometry, the eccentricity cannot be negative. I know that the shape of the orbit depends on the energy, i.e. the velocity. I suppose it's a matter of speed. For circular, neglecting air resistance, v=Failed to parse (syntax error): {\displaystyle (GM/r)^½} Kissnmakeup (talk) 18:57, 25 July 2008 (UTC)

Cannonballs are never parabolic, although the difference from parabolic flight (ignoring air drag) is extremely small in most everday circumstances. They're always elliptical, basically because the Earth surface is not flat. If the Earth was flat, then they would be parabolic.- (User) WolfKeeper (Talk) 19:11, 25 July 2008 (UTC)

(Actually, if you're being really pinickity, they're not elliptical either, they would only be elliptical if the Earth was perfectly spherically symmetric, but let's not go there.)- (User) WolfKeeper (Talk) 19:11, 25 July 2008 (UTC)

FWIW eccentricity is not primarily a matter of speed, although it is involved, for any given speed it's a matter of the direction at a point on the orbit.- (User) WolfKeeper (Talk) 19:11, 25 July 2008 (UTC)

More orbits

Thank you.

I'm studying for the GRE Physics test, and I have a question about a problem on an old practice test. Would you want to give it a whirl?

The problem is this (Test 9677 problem#66):

When it is about the same distance from the Sun as is Jupiter, a spacecraft on a mission to the outer planets has a speed that is 1.5 times the speed of Jupiter in its orbit. Which of the following describes the orbit of the spacecraft about the Sun? A) Spiral, B) Circle, C) Ellipse, D) Parabola, E) Hyperbola.

ETS says (E) Hyperbola. I say bull. According to Mechanics by Keith Symon, the shape of the orbital depends on the total energy, not just the kinetic energy. The mass of Jupiter is about 1.8 x 10^27 kg, while the Mars Global is about 1030 kg. This is a massive difference in potential energy. Does it not matter in light of the mass of the Sun which is 1048 Jupiters? I would think that it would matter. I mean, multiply it out. GMm/r Kissnmakeup (talk) 20:55, 25 July 2008 (UTC)

Because you told me it's homework, I'll give you just one hint: it's the specific total energy (total energy per kg) that matters, not the total energy.- (User) WolfKeeper (Talk) 21:09, 25 July 2008 (UTC)

It's not homework. This is a practice test from 1996, with published answers. I just don't agree with the answer that ETS published for it. But, I really appreciate your response. Thanks. Kissnmakeup (talk) 22:14, 25 July 2008 (UTC)

Really, it's a specific energy/escape velocity question, right? The mass of the body itself cancels out in the equations. (Actually the deep knowledge is that it's to do with Galileo all things fall the same under gravity.) The escape velocity at Jupiter's orbit is 18.5 km/s, and the orbital velocity is 13.07 km/s. Since 1.5*13.07 > 18.5 then it's a hyperbola.- (User) WolfKeeper (Talk) 22:32, 25 July 2008 (UTC)

Duh.

Thanks. Was it Galieo or Newton? Regardless, I was not happy with that classical mechanics book when I took the class, and I'm even less happy with it now. I'm getting a different one, and maybe even an astrodynamics text. All this orbit stuff is making my head spin.Kissnmakeup (talk) 00:57, 26 July 2008 (UTC)

Galileo. You probably heard the (probably apocryphal) story about cannonballs and the tower of piza etc. etc.- (User) WolfKeeper (Talk) 01:06, 26 July 2008 (UTC)

Internal Combustion Engine page

Hey sorry I reverted your edit to the summary, but I put it back. I changed it to the one that included the components used and I hope we can both later add to the article without conflict. You're English has gotten a lot better from what I've seen lately. :-) Sincerely, InternetHero (talk) 09:15, 31 July 2008 (UTC)