Johnny5

Maybe this will help:

 

Assuming the force does not act through the centre of mass.

 

All of the force will also go into rotation. The moment will depend on the magnitude of the force and the "lever" due to the alignment of the force wrt the centre of mass.

Yes that helps' date=' but can you prove that it is true for me? Will energy be conserved?

Let the applied force at the end of the bar be F, perpendicular to the bar.

The torque is found from this:

[math'] \tau = torque = \vec r X \vec F [/math]

The center of mass is going to be accelerated. And you are telling me the acceleration of the CM of the bar in the original rest frame of (me +bar) is found from

F=ma

where m is the mass of the bar.

Does this violate conservation of energy?

It takes more energy to maintain the force.

 

If you and I were in outer space with bars and we were equally powerful and I pushed with all my might through the centre of mass and you pushed with all your might on the end as described on an equivalent bar then I would exert more force.

Can you do this with numbers?

Suppose we push on the bar in opposite directions, both of us through the center of mass of the bar.

We push equally hard, because we are equally strong.

Now focus on things in the center of mass frame of the bar.

The bar remains at rest in this frame.

You move away from me, and I move away from you, and the bar remains at rest in the CM frame.

The force upon me is 2F, and same on you.

I exterted a force on you of F, and an equal force was reflected back upon me, and you exerted a force on me of F, and an equal force was exerted back upon you, so the total force upon me was 2F, and letting M denote my mass, my acceleration in the CM frame (bar at rest) is found through:

2F = M a

Now, move me to the edge of the bar, and we both push simultaneously, equally hard. You say you exert a greater force. Explain this to me, i expect you to say something about torque.

All [/b'] of it will go into translation.

But my intuition says the bar is gonna spin? What on earth???

Here is how I am viewing this.

Let there be a master frame, in which the center of mass of the universe is at rest.

Let my center of mass, and its center of mass be at rest in the master frame, and furthermore neither me, nor the bar is spinning in the master frame.

Then I push the end of the bar. Now, the center of mass of the bar has accelerated in the master frame, but my intuition also tells me that the bar will be spinning in the master frame. What, if anything, is my intuition doing wrong?

I seem to be convinced, that the bar will now be spinning in the master frame, as well as having its center of mass be translating.

It will spin and translate (wave good-bye to it)

 

It will move away in the direction of the force and you will move in the opposite direction. Depending on the alignment of the force with respect to your centre of mass you may spin also.

Yes, it will move away, I know that. My center of mass, and it's center of mass will separate, in the center of mass frame (me+it) I know this (assuming Newton's third law true). But, how can one applied force F, at the end of a rigid body, be equivalent to the same force acting in the same direction at the center of mass, plus a rotational moment? <--- thats what i don't get.

Doesnt that violate conservation of energy?

What I was trying to show here is that any force acting on a rigid body that is not acting through the centre of mass is equivalent to an equal force in the same direction acting through the centre of mass plus[/b'] a rotational moment.

See I don't think this is right. Won't some of the applied force go into translation, and some go into rotation?

What I was trying to show here is that any force acting on a rigid body that is not acting through the centre of mass is equivalent to an equal force in the same direction acting through the centre of mass plus[/b'] a rotational moment.

So then you are saying that it does spin?

Regards

I think this is where you are going wrong. ALL of the force acts to translate the centre of mass.

I don't follow. I am picturing a bar in space. I am floating next to it, and am at rest with it. Then I use my finger to push the leftside of the bar, in a direction perpendicular to the bar. That bar is going to start spinning isn't it? If not, then yes I am doing something wrong. Why won't the bar start spinning?

Earlier in this category, I began a thread on Grand unification. I asked what kinds of things must the final supertheory of physics explain, and I got some nice answers. Some things there, I doubt I will ever understand. I am posting this thread here, because it seems the natural place for it.

I've been reading a lot of things going on in physics today. There are so many physicists coming from so many different angles at "who knows what." They cannot all have the right approach.

I've seen things on M-theory, which I have no clue about.

I've seen things on string theory, which I also have no clue about.

I've seen physicists poking fun at each other, for not having a clue about what each other is saying.

I have seen people quantizing space, using Planck length. I'm not sure they know what they are doing either, they still use the notion of continuity I think, but I can't say for sure because I don't know what they are doing.

I am looking at the whole problem epistemologically.

That means that there are certain rules one must follow in order to know something.

Aristotle had a lot of helpful information on this, believe it or not.

To his end, he focused on what he called axioms.

Two thousand years later, no one knows what he meant.

When I first seriously studied quantum mechanics, I became aware that there were five axioms for quantum mechanics. I still don't understand them, and doubt that they are necessary and sufficient to lead to the Schrodinger equation, which I did understand... at least to my own satisfaction anyway.

I have a question, which is hard to verbalize.

Let us suppose that there are N physicists in the world, trying to "unify physics." They all have different approaches, and theories, and none of them are where they want to be.

Let us extrapolate something from them all.

What is the portion of knowledge of physics, which the majority of them have in common?

In other words, what is their collective knowledge?

I don't want a list of ten million facts. The way Euclidean Geometry worked was this, you start out with five axioms, and some common notions, and then deduced a few hundred or so theorems. The Greeks were pretty confident that the five axioms led to all true statements about three dimensional space.

So what are the Axioms of physics?

I guess that's my question.

It seems to me, that once you choose a model, the axioms sort of just flow from having the right model, so what is the right model?

Or to put my whole question another way, suppose that I was a twenty fourth century student, about to begin learning physics, after physics had already been unified. What would be the main thing that I would have to learn?

I don't know if anyone here can actually answer this question, I just thought I would ask, at least to read what others have to say on the matter.

Thanks

PS: If I were going to answer my own question, I would start off like this:

Axiom I: Simultaneity is absolute.

Axiom II: Space is three dimensional.

Axiom III: Time is quantized.

Axiom IV: F= d(mv)/dt

Axiom V: The total inertial mass of the universe is conserved.

Axiom VI: The total electric charge of the universe isn't conserved.

Axiom VII: Quantum electrodynamics is correct.

Axiom VIII: (Conservation laws)

Axiom IX: p = hbar k

You get the idea. What would be added, what would be subtracted from the list which I gave?

Isn't it the inverse squared laws which lead to stable orbits?

I've never actually proven this in enough detail. I've read other people's proofs but never attempted to construct my own, maybe now is a good time to do so.

When you say stable orbits, I presume you mean ellipses (circle also an ellipse). But also, inverse R^2 force can lead to hyperbolic orbits (not stable).

So first I guess you have to prove that only inverse R^2 forces can lead to conic sections, so that only inverse R^2 forces can lead to the stable ellipse orbits. I have Feynman's gravity lecture on tape, and he uses some very arcane properties of ellipses known to the ancient Greeks, but I couldn't even follow his entire proof, because he fudged in at least one area (used something without proof). This type of proof avoids using analytic geometry, and avoids using vector calculus, and so is the most intuitive type of proof, because you can visualize the motion so well.

Let me ask you something whose answer should be simple... what is the quickest way you know of to prove that only inverse square forces lead to ellipses?

Thank you very much again.

Regards

The force on the leftmost particle (upwards) is equivalent to having the force on the middle particle (upwards) plus half of that force on the leftmost particle (upwards) plus half of that force on the rightmost particle (downwards) which resolves to a translational F=ma plus a rotational moment.

 

Ok' date=' just to make sure I followed you, let the net applied force to the system have a magnitude F. Suppose that X is the portion of F that goes into translating the center of mass. Let the remainder be Y. Therefore, X+Y=F. And you are also saying that X=Y.

In order to figure out how the entire line mass moves (not just the center of mass of the line mass) we can break the force F into three forces. One with magnitude X, which pushes the CM upwards, and then one which pushes the leftmost particle up (which will have magnitude Y/2), and a third force which pushes the rightmost particle DOWNwards.

So if I understand you correctly, you are saying that what happens by applying just one force F to the leftmost particle, will be the same thing which happens if three forces act simultaneously on the object at different places. On with magnitude Y/2 upwards on the leftmost particle, another with magnitude Y/2 downwards on the rightmost particle, and a third with magnitude X upwards on the center of mass particle (assuming there is a particle there (and there isn't because the number of protons is even but ignore this for now), and Y/2+Y/2+X=F. <--- Is this what you are saying?

Assuming that is what you are saying, what is the simplest way to mathematically check if what you are saying is true?

Would you switch to watching the motion in the CM frame (which translates with the body after the push), and prove that in this frame there is only pure rotation?

Can you do a mathematical analysis of the problem in various frames of reference? Intuitively you know you can break the force into three, but exactly how do you infer that from Newton's laws? I have so many questions about this one problem. Next you say:

 

As the "bar" accelerates the point of contact of a constant upward force on the original particle will shift to the right (with the particle) reducing the rotational moment. It will eventually oscillate left and right (constant frequency) while continually accelerating upwards at greater velocity.

Can you explain this part in greater detail? What did you assume was responsible for the applied force? A particle? Another line mass? Also, what oscillates? Are you treating the line mass as purely rigid? Or almost rigid?

It's not an easy problem, perhaps not even well posed, but still I want to work on this.

Thank you very much, regards.

Which means that they have one property in common (zero rest mass). One shared property is no evidence of being the same thing.

 

The proton and positron share the same property of charge' date=' that doesn't make them the same thing. The electron and muon share three common properties, even that does not make them the same thing.[/quote']

 

Agreed, but something is still telling me that grand unification lies in some kind of simplification. Just wondering if its here. I would think that particles should be classified according to their mass.

Well well... I'd never have guessed... the compass needle is effected by it' date=' not much, but a tiny bit... the field is too small to effect 0.5g of metal, too small to effect even 1ml of ferrofluid, but it does effect a compass needle.

 

Now the NIB we're talking about is big, about 1.5inch or 4cm and its powerful! So what causes 99.9% of the field to be lost through the piece of metal?

 

And then when you place a 2nd piece of metal there the field becomes full strength, its soo weak, can't even effect 1mm of ferrofluid, but it does effect a compass.... what causes such a massive magnetic field blocking? (and why when the 2nd piece of metal is added does that remove the shielding effect?)[/quote']

 

I realize that this is your whole question, the one you want answered. When you bring the second piece in, do you place it in contact with the magnetic, or as you bring it in, does the field strength slowly increase (watch the needle)

 

Regards

Conservation of momentum

Ok, explain this mathematically please. I want to see how you analyze the problem.

Thanks

All [/b'] of the force will move the center of mass in the upward direction

Why?

Only [/b'] in the direction of the force and according to F=ma

Ok lets try something.

Suppose the mass of each particle is 1.67 x 10^-27 kilograms, and that there are 100 such particles at rest with respect to each other, but held together firmly, so that the line mass is rigid.

Then there is some kind of interaction with the leftmost particle, and the force there has magnitude F, and the direction is up (perpendicular to the line charge).

Describe for me the path which the leftmost particle takes, as a function of time.

I understand that you are going to take some of the applied force, a fraction of F, and say that this portion of F acts as if it pushed the center of mass upwards. Therefore, the center of mass of the system will have some upwards motion.

The rest of the force will go into causing some rotation.

But here is where I am going with this. I want to watch your solution to the problem, and see if you use the concept of an inertial wave.

Regards

No' date=' there is no magnetic field under there, I am positive, or at least if there is it can't even effect 0.5g of metal (which is attracted to magnets)... if there was a magnetic field it'd be as strong as the air is solid.... I am sure its not there.

 

Why would the addition of a second peice of metal double the field strength?[/quote']

 

Have you tried using a compass needle?

 

As for it doubling the field strength, I don't really think it would, just asking if you thought so. It's very hard without having everything in front of me to tell you what's going on, but I can't help but think this is some kind of basic problem in magnetism.

 

Regards

Suppose you have a line mass distribution

..........................................

and something pushes the left most particle, leftmost side. How does the center of mass move?

ok....can you please go back to answering the intial question in the thread??? pleaseeeeeeee

The term 'flat' has to do with whether or not the universe will expand forever, or not.

Flat universe

Then you will accelerate the centre of mass according to F=ma

Mmm in what direction?

Regards

Well if you read post #16 it seemed to rule out the possibility of it being the metal, but logic rules out an acrylic glue causing this... it can't be the magnet itself..... I dont know!!!!

Could it be the case that there was initially a weak magnetic field under the metal, and you were unable to measure it, and then when you brought in the second piece of metal, you doubled the field strength underneath, and now could measure it?

Regards

F=ma

 

It does not matter if it acts through the centre of mass. It will accelerate the centre of mass of that "body" in the direction of the force. It does not matter what shape' date=' rigidity or "coherence" the body, gas cloud or "deformable object" is getting "accelerated". The centre of mass will accelerate according to F=ma, in the direction of the force [b']except for relativistic effects[/b]

 

If it does not act through the centre of mass it will also impart a torque or moment on the body.

What if the applied force is 90 degrees relative to the normal drawn through the point of contact? What then?

'']There is currently no basis on which you can say they (gravitons and photons) are the same thing.

Well yes there is, they have the same special fundamental speed, with respect to the source.

Regards

 

Also read post #15 and #16.... they give more interesting information.

In post #15 you mention a "magnetic field blocker." What do you think is blocking the magnetic field. The metal? The glue?

Regards

If everything started out with a big explosion, why would it be anything other than spherical?

If, by the word 'universe' we mean everything with inertial mass, and everything without inertial mass, then the universe cannot have a shape.

Saying everything without inertial mass, is a fancy way of referring to the vacuum. 'Space' cannot be spherical, it cannot be cylindrical, it cannot be a cube.

Regards

Thank you

You're welcome.