Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 06/04/2018 19:01:02
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I understand that the why's of physics are not usually answered. Why not have a stab. Just for the hell of it.
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I thought about this not that long ago. I considered the possibility that inertia may be a result of the fact that kinetic energy transfer takes time. The stronger the field, the less time it takes for a given body to be given a certain amount of kinetic energy (which results in greater acceleration), but it always takes some finite time for that to occur due to the limited speed of light.
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I was thinking about motion in the absence of external forces but I do take your points on board.
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Why do objects move inertially according to Newton's laws?
Because on a small scale (eg a billiards table), space is effectively flat and Newton's laws of linear motion are identical to General Relativity.
On a slightly larger scale (eg the Solar System, but outwards from the orbit of Venus), space is effectively flat, and Newton's laws of orbital motion are identical to General Relativity.
It is only when you observe the orbit of Mercury in great detail over a period of centuries that the slight curvature of space near the Sun becomes detectable as a deviation from Newton's laws of orbital motion.
When you are dealing with orbiting pulsars, the curvature of space is much greater, and deviations from Newton's laws are detectable in a couple of years.
See: https://en.wikipedia.org/wiki/Hulse%E2%80%93Taylor_binary
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I was thinking about motion in the absence of external forces but I do take your points on board.
If there are no forces at work slowing an object down, then it seems to me that there is no reason for it to slow down.
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But why doesn't the object slow down? It could be that there is a non accelerative force acting upon it. That may sound odd but think about it a bit. If this non accelerative force is imbalanced in a particular direction then it would be indistinguishable from gravity.
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Say we have an object that is stationary. All the fields around the object are balanced and no motion is imparted to it. If the object is moving then the fields in the direction of motion are blue shifted and impart an amplified force to the object. The fields away from the direction of travel are red shifted and apply less force to the object. The fields perpendicular to the direction of travel are balanced and so cancel. The motion will continue in the direction of the blue shift.
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It can be thought of like the situation in a gravitational field where a stationary object is on a plateau and a moving object is on an inclined plane.
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a moving object is on an inclined plane
A ball on an inclined plane will accelerate.
This would violate Newton's law that says an object will continue at rest or with a constant speed unless acted on by an external force.
See: https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion
One of the factors at the heart of Newton's physics is conservation of momentum, which is implicit in statements like every action has an opposite & equal reaction. Noether's theorem provides a mathematical basis for conservation of momentum.
See: https://en.wikipedia.org/wiki/Noether%27s_theorem
PS: It is interesting that the mathematical derivation of conservation of momentum contains an assumption of "1-dimensional time"!
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Yes the inclined plane and gravity was a terrible idea since it involves acceleration. But gravitational free fall does a good job of seeming like an inertial frame very locally. If we were to construct an inclined surface whose shape guaranteed constant velocity that would be interesting. Also probably impossible.
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If the object is moving then the fields in the direction of motion are blue shifted and impart an amplified force to the object. The fields away from the direction of travel are red shifted and apply less force to the object.
I follow the reasoning, but what are the "forces"?
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As has been said @Kryptid , why would it slow down unless a force makes it do so. If space had a friction or drag effect that would do it, so perhaps the answer is that space does not have a drag effect on mass.
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perhaps the answer is that space does not have a drag effect on mass.
This would certainly seem to be the case, which is why I'm hoping Jeffrey will identify the "forces".
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My musings contain an inaccuracy. Force is defined as a change in inertia. One of the forces that change inertia is gravity. A maintenance of constant velocity cannot have a force involved. Yet free fall has a force involved and acts like inertia. I am still thinking it through. When it becomes clearer I will give an update.
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Force is defined as a change in inertia. One of the forces that change inertia is gravity.
Typo? "Inertia" is not a word commonly encountered in physics.
Do you perhaps mean that "Force causes a change in momentum" or "Force causes a change in acceleration"?
Inertia is a property of mass, and represents the degree to which an object resists a force, via Newton's F=ma.
A change in the mass changes inertia.
A change in the gravity changes the acceleration, which is why Newton described it as a distance-dependent force.
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I understand that the why's of physics are not usually answered. Why not have a stab. Just for the hell of it.
Nobody knows. That's why It's called Newton's Laws rather than Newton's Theorems.
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@evan_au No I meant inertia. The force makes an object move faster. It's inertia is changed and it now maintains the faster speed once the force is removed. However it now acts like its inertial mass is increased and progressively more force is required to move it. This tends towards an infinite force which indicates that the inertial mass tends towards an infinite value. This isn't a tenable explanation. We must be missing something fundamental. If you see an infinity then the solution has to be wrong.
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@evan_au No I meant inertia. The force makes an object move faster. It's inertia is changed and it now maintains the faster speed once the force is removed. However it now acts like its inertial mass is increased and progressively more force is required to move it. This tends towards an infinite force which indicates that the inertial mass tends towards an infinite value. This isn't a tenable explanation. We must be missing something fundamental. If you see an infinity then the solution has to be wrong.
The faster it moves the harder it is to accelerate. The increase in inertial mass is due to the properties of spacetime. If you follow the derivation of this property you'll see how its determined to be so.
The solution is not wrong because you know of an infinity. A body with finite proper mass cannot move at the speed of light. Therefore the inertial mass always has a finite property. You're thinking of the fact that there is no upper bound to this mass, i.e. as v -> c the mass increase without bound. Recall how the term infinity is defined. And there is no problem in physics with infinities. In fact our very own universe appears to go on forever, i.e. is infinite in size.
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The increase in inertial mass
Ah... I see now that Jeffrey was talking about relativistic conditions, where inertial mass is not a constant.
But under relativistic conditions, an object no longer obeys Newton's laws
- or you have to bend F=ma to say that m varies with velocity or gravitational potential - and that the time you use to measure acceleration also changes.
- these are not modifications that Newton would recognize!
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Ah... I see now that Jeffrey was talking about relativistic conditions, where inertial mass is not a constant.
But under relativistic conditions, an object no longer obeys Newton's laws
- or you have to bend F=ma to say that m varies with velocity or gravitational potential - and that the time you use to measure acceleration also changes.
On the contrary. Newton's Laws are still valid. It's a common misconception that Newton defined F = ma. That was a relation proposed by Euler, not Newton. Newton defined force, essentially, by F = dp/dt. And his third law always holds when the forces are contact forces.
To be precise, Newton stated that force is proportional to change in momentum and moment he defined as density times speed.
- these are not modifications that Newton would recognize!
When used correctly, sure he would. :)