Mach Effect progress

[chrismb]

I am disagreeing with your statement that there is no change in momentum unless there is a displacement. It seems your argument of an immovable object rests on this statement. Just because it is immovable does not mean it's momentum cannot change due to a force, a priori at least.

I beg to ask you to examine your own statement: If a force is applied to an object that alters its momentum, if it has not changed its displacement from the origin of that force (i.e. nothing has changed) then by what measure are you judging its momentum to have changed?

You might well choose to calculate a change of momentum as force wrt time, it may well be convenient to do so for many situations, but that is not the only way to do it. It is quite quite impossible for a force to be applied to a [free] object that is not then displaced by that force (unless it was rigidly attached to the inertial frame in which the force originated).

(The momentum, energy, force and acceleration (thus, time and displacement) are all interconnected, and you can pull any three of those out in isolation to look at, if you choose to. But it will tell you no more than any other three, as they are all equivalent at a lower level.)

[kcdodd]

If you apply a force for a specific amount of time to any object, its momentum will change by a specific amount. That is by definition.

This immovable object is your construct. I cannot answer what momentum means if the object is not moving. That does not mean the concept of force is ill-define, it means your object is ill-defined.

One way to do it is to look at the limit as mass goes to infinity. Such an object can exchange momentum, but not energy.

[KitemanSA]

If you apply a force for a specific amount of time to any object, its momentum will change by a specific amount. That is by definition.

Not quite. You must apply a NET force, an unbalanced force for a time to achieve a momentum change. The statement "immovable" implies that ANY force applied will by necessity result, somehow, in a counter-balancing force. No unbalanced force, no momentum change. QED.

Now, if a tree falls...

[kcdodd]

Define "somehow", then you can say QED. lol.

I can push on myself without moving, but I am not immovable.

Maybe if chrismb can clarify how such a thing would exist under his understanding.

[KitemanSA]

You caught that did you? Well done.

But you a obviously trying to weasel your way out of answering my totally valid point about the tree. Slippery as an eel, aren't you!

[kcdodd]

I would guess "If a tree falls..." = 7.32.

[DeltaV]

However, I do see the confusion that talking about 'forces' and 'momentum' can cause, which doesn't happen if you stick with 'energy'.

Which has the same dimensions as torque, or force*distance.

[tomclarke]

Define "somehow", then you can say QED. lol.

I can push on myself without moving, but I am not immovable.

Maybe if chrismb can clarify how such a thing would exist under his understanding.

Agreed.

I beg to ask you to examine your own statement: If a force is applied to an object that alters its momentum, if it has not changed its displacement from the origin of that force (i.e. nothing has changed) then by what measure are you judging its momentum to have changed?

Well an impulse will change momentum without changing displacement. You judge the changed momentum by observations over an extended period of time after the change. Indeed without such a period energy (and hence in an isolated system momentum) cannot be measured. But introducing QM here is not needed. The point is that velocity (and hence momentum) is the first derivative of displacement.

I take your point that if you can push yourself away from a thing that doesn't move then you've created momentum spontaneously, which is not possible. But that's not my point. What I'm saying is that if you find something 'stationary' then it is not so much that you can push yourself away from it, so to speak, but that you just can't find a way to apply a force at all! It won't have any 'handles' to push or pull on!

Words are poor tools to describe physics.. So forgive me if I am misunderstanding you. For better understanding, you need less ambiguous statements (not words!).

You are saying that acceleration is only possible for objects in a frame of reference other than that given by the universe COM. This is radical, since it breaks special relativity. It is also unclear how this works. Suppose your "immovable object" has a small movement relative to COM frame. Does it behave normally, or with some increased inertia?

As kcdodd has pointed out, the existence of your "immovable object" creates grave problems. Difficult to see what mechanism would prevent lack of conservation of momentum.

Then you start talking about photons. This is less clear, and therefore less obviously inconsistent. But a transformation in which c -> 0 is pretty radical, worse than the > c <--> < c transformation (which is possible) because 0 is not, according to principle of relativity, well defined.

I have stayed clear of the above argument since it did not seem to be going anywhere. Now it IS going somewhere i am registering my disagreement with the destination!

Best wishes, Tom

[chrismb]

If you apply a force for a specific amount of time to any object, its momentum will change by a specific amount. That is by definition.

The difficulty here is that I am trying to say that it is really difficult to explain some things by the derivative notions of force and momentum, and then I am being asked to explain things in force and momentum! It should be possible because what I am saying is not 'wrong' and conflicting with these notions, only an alternative description of them, so I will try but if it's not particularly intuitive then all I can say is "I told you so!".

In normal parlance; a reaction force is being exerted by the table in front of me up on the monitor, and the monitor is imposing a weight force on the table below. Yet... no change of momentum!!!???

Momentum relates to the relative motions of the centres of mass of two groups of fundamental particles, which you choose to define as groups. If you look at a volume of gas, you could say "there is no momentum present for this volume of gas". I could equally say "ah! half of the gas molecules are going left and half are going right, so there is momentum present" and, seemingly instantaneously I have created the notion of momentum where you saw none. The net momentum is always zero, of course. So my point is that if you find something fixed in the centre of mass of everything, then if you could move it so you would *then* have compromised the conservation of momentum.]

The net momentum of the CoM of the universe is our momentum ground zero!

[chrismb]

One thing I have been caused to contemplate here is the behaviour of rotating things.

Let's try to use existing terms to describe two equal masses tied together with a string (of neglible thickness and mass, as the exam questions go) and that they are rotating around a point which is the centre of the string.

We can conceptualise vectored momentum in each axis for each mass separately. But it is clearly more informative to note that there is no net momentum, else we end up with sinusoidal radial forces in each axis to cope with and, blimey, you can go write some equations for that!....

What I have come to realise is that where I talk about a change of configuration for location, in a given instance it may be that there is no radial change of location with respect to the CoM of the universe. So what I have described doesn't exclude (from this notion of 'stationary') the scenario is a mass that is moving azimuthally around a CoM, but not radially.

I think, then, that azimuthal motion around a CoM should also be regarded as 'stationary', for the purpose of analysing d[entropy]/dx, provided there is no radial motion.

[chrismb]

You are saying that acceleration is only possible for objects in a frame of reference other than that given by the universe COM.

Depends what you include in your frame of reference. A spaceman sitting in a small capsule out in space who jumps across his spacecraft could cause it to appear [from the outside] to slow down momentarily and then speed back up once he hits the other side. If you pick out objects within the inertial frame then you could describe momentum being transferred. But the momentum wrt CoM hasn't changed during that jump.

The capsule CoM cannot accelerate by itself, it needs something else external to the masses in that inertial frame to cause the CoM to accelerate. It is trivial to then say that the CoM of everything cannot be accelerated. It is a unique CoM because it is defined by having nothing else to react against.

I don't see any conflict with [the outcomes of] Relativity, nor do I see any with Newtonian mechanics. What I see is that these three ideas are based on different axioms. The fact that they lead to the same outcomes doesn't help determine which axioms are correct. You can get the right answer with wrong starting assumptions. I could have an axiom that the God Sol likes to use blue paint to colour the sky every morning, and at night he sometimes runs out and has to use red. I get the right answer - the sky is blue - but that doesn't prove my axiom.

[KitemanSA]

I take your point that if you can push yourself away from a thing that doesn't move then you've created momentum spontaneously, which is not possible. But that's not my point. What I'm saying is that if you find something 'stationary' then it is not so much that you can push yourself away from it, so to speak, but that you just can't find a way to apply a force at all! It won't have any 'handles' to push or pull on!

OR if the item is "unmoved" SOMETHING ELSE must have put the same impulse into it in the opposite direction and attained the opposite momentum. Momentum IS conserved.

[tomclarke]

You are saying that acceleration is only possible for objects in a frame of reference other than that given by the universe COM.

Depends what you include in your frame of reference. A spaceman sitting in a small capsule out in space who jumps across his spacecraft could cause it to appear [from the outside] to slow down momentarily and then speed back up once he hits the other side. If you pick out objects within the inertial frame then you could describe momentum being transferred. But the momentum wrt CoM hasn't changed during that jump.

The capsule CoM cannot accelerate by itself, it needs something else external to the masses in that inertial frame to cause the CoM to accelerate. It is trivial to then say that the CoM of everything cannot be accelerated. It is a unique CoM because it is defined by having nothing else to react against.

I don't see any conflict with [the outcomes of] Relativity, nor do I see any with Newtonian mechanics. What I see is that these three ideas are based on different axioms. The fact that they lead to the same outcomes doesn't help determine which axioms are correct. You can get the right answer with wrong starting assumptions. I could have an axiom that the God Sol likes to use blue paint to colour the sky every morning, and at night he sometimes runs out and has to use red. I get the right answer - the sky is blue - but that doesn't prove my axiom.

Right - but what you now say is different from what you seemed to say before. You are saying that a closed system with zero momentum (in the CoM inertial frame) will always have zero momentum.

Indeed that is true, and it is a special case of conservation of momentum. Nor does it imply anything above what is given by conservation of momentum.

Previously you were saying that an object stationary wrt the CoM of the universe would remain stationary wrt this, even if acted on by some other) object.

If however you are merely saying that the CoM of the universe (which is an abstract position, not an object or system of objects) cannot be accelerated, that is fine.

So maybe we are back to this thread not going anywhere fast, and I can bow out of it!

Best wishes, Tom

[icarus]

Caution: center of mass of the universe is a high octane concept.

Maybe I shouldn't have left it in eager hands with a box matches, like Mach-Effect thrust, lying around?

Have fun with it, but try not to blow yourselves up. I won't provide a link to an Amazon website where you can purchase a reference text. Because there is none, as far as I'm aware .... and I'm quite aware.

[cuddihy]

Caution: center of mass of the universe is a high octane concept.

Maybe I shouldn't have left it in eager hands with a box matches, like Mach-Effect thrust, lying around?

Have fun with it, but try not to blow yourselves up. I won't provide a link to an Amazon website where you can purchase a reference text. Because there is none, as far as I'm aware .... and I'm quite aware.

The references are all free:

http://articles.adsabs.harvard.edu//ful ... 4.000.html

http://physics.fullerton.edu/~jimw/general/