Why does an object start to fall under gravity

[Dale]

I am trying to explain motion from energy only and it works for every observable situation. (by works it agrees with forces produced by interaction with fields.)

The point I have been trying to get across to you is that your explanation is wrong (or at least incomplete), even when you are dealing with a case that has motion. You have to do more than what you are suggesting. Specifically you have to use energy to construct the action then use the principle of least action to analyze the motion. Otherwise even for a moving object you cannot get the right path.

For example, given an object of 1 kg mass in a 1 g uniform field, what is the trajectory if it has KE = 100 J and PE = 0 J initially?

If that is all that you are given then you cannot solve for the trajectory.

Even if you are also given an initial velocity, how does simple conservation of energy tell you that you get a parabolic path. Why not just a straight line path with the speed determined by energy considerations? You need something additional, the principle of least action.

Since your explanation doesn't work for the moving case, you shouldn't be surprised that it doesn't work for the stationary case either.

 

[Adeste]

The point I have been trying to get across to you is that your explanation is wrong (or at least incomplete), even when you are dealing with a case that has motion. You have to do more than what you are suggesting. Specifically you have to use energy to construct the action then use the principle of least action to analyze the motion. Otherwise even for a moving object you cannot get the right path.

Many thanks for your efforts particularly if I am frustrating you.

My apology my opening statement is misleading

I agree 100% with what you say I am not trying to replace mechanics (forces) with potential changes for moving objects. Therefore I am not trying to prove a path or trajectory mechanics both works and is very sophisticated at this.

My only concern is if a particle starts moving from rest ie stops remaining stationary. My model predicts it does not move, Any path would prove the model wrong. I am only trying to prove my model wrong. Any path would do.

To try to answer your point

I feel the Principle of minimum energy or 2nd law of thermodynamics constrain the particle to move to a lower potential. If the field is linear this will be perpendicular to the field (thus in the direction of the force). Such would be sufficient to prove my model wrong.

again I apologise if my comment seems condescending it is just that I am working from first principles only.

 

[Adeste]

No, you can never break energy conservation, neither in classical mechanics nor in quantum mechanics.

Yes I agree as mechanics is based on observation this not possible. The particle in my model has I believe never been observed and possibly can never be observed.

Not sure if I am allowed to do this but there is a thread which although it is inconclusive indicates that conservation of energy cannot be observed if Δt.ΔE is small enough.

https://www.physicsforums.com/showthread.php?t=120224

Therefore I feel my causality which you object to is at least feasible or more importantly not yet shown to be false.

Thanks very much for your thoughts

 

[Adeste]

Work is being done, PE and KE are changing. Of Course force is involved.

I really can't see your point, here. There are forces involved, changes of energy and fields. The velocity - whether zero or not - is irrelevant.

What "explanation"? You just seem to be asking a question, involving some random ideas. The 'correct' bits of your 'model' are correct but what's new?

I agree work done is done as there is a change in energy.
and I agree mechanics use F.X=Work Done

To answer your second question what is new.

First I do not know if what I suggest is new.

However it seems mechanics has been built on theories which are then proved true or false by observation. Hence mechanics is an excellent model for observable objects.

I believe as it is impossible to observe an object with velocity = 0 only objects whose average velocity = 0 by this I mean all observed objects have internal energy. I think of them as having little bits vibrating around ( quarks molecules ...whatever)

However mechanics takes these observations and applies them to a theoretical unobservable particle and assigns such particle with velocity =0

In short I am saying mechanics has not been validated for this theoretical state an unobservable state of a particle.

I am unsure of the ramifications of this.

My apologies again for lack of scientific terminolgy

 

[Dale]

I agree 100% with what you say I am not trying to replace mechanics (forces) with potential changes for moving objects. Therefore I am not trying to prove a path or trajectory mechanics both works and is very sophisticated at this.

My only concern is if a particle starts moving from rest ie stops remaining stationary. My model predicts it does not move, Any path would prove the model wrong. I am only trying to prove my model wrong. Any path would do.

Then I don't understand the point. You already seem aware that your model doesn't correctly describe the motion of a general moving particle, so why would you be at all concerned that it also doesn't correctly describe the motion of a stationary particle?

It is possible to use energy principles only to correctly describe a particle in general, you just have to use the Lagrangian approach. You should hold off trying to invent your own approach until you have actually learned Lagrangian mechanics.

 

[Adeste]

Then I don't understand the point. You already seem aware that your model doesn't correctly describe the motion of a general moving particle, so why would you be at all concerned that it also doesn't correctly describe the motion of a stationary particle?

It is possible to use energy principles only to correctly describe a particle in general, you just have to use the Lagrangian approach. You should hold off trying to invent your own approach until you have actually learned Lagrangian mechanics.

My model does absolutely describe the motion of a general moving particle.

I apologise if Lagrangian mechanics shows this. I leave you to judge.

However I conclude that gravitaional force is not equal to mg but ma where a is the actual acceleration of the actual particle not a theoretical value.

To be precise f=mg does not work for a stationary particle but does at all other times.

 

[DrewD]

My model does absolutely describe the motion of a general moving particle.

I apologise if Lagrangian mechanics shows this. I leave you to judge.

However I conclude that gravitaional force is not equal to mg but ma where a is the actual acceleration of the actual particle not a theoretical value.

To be precise f=mg does not work for a stationary particle but does at all other times.

Your "model" claims that particle with zero internal energy will never accelerate from rest. Depending on how you define internal energy, this is either not true, or it is impossible to have a particle with no energy (if you want to consider rest energy of a particle in SR). If you have a theory of physics that predicts things that are never observed, your theory is wrong.

If the force due to gravity is the mass times the actual acceleration of the particle, then a particle at rest would have no weight. Again, this is pure silliness. If there were some observation of this, then maybe it would be interesting, but there hasn't been and this isn't.

$f=mg$ does work for a stationary particle. The force do to gravity of a particle at rest is $mg$ which opposed by some normal force that is equal and opposite and therefore the sum of all forces is 0.

Your "model" is not a model because it does not describe anything real. You are trying to find a way to solve a problem that arises because of a false assumption. The resolution should be obvious. I'm pretty much done with this thread since you appear to be sticking to your guns and uninterested in actual study. Your "model" predicts things that don't happen. Give it up.

 

[Adeste]

Your "model" claims that particle with zero internal energy will never accelerate from rest. Depending on how you define internal energy, this is either not true, or it is impossible to have a particle with no energy (if you want to consider rest energy of a particle in SR). If you have a theory of physics that predicts things that are never observed, your theory is wrong.

If the force due to gravity is the mass times the actual acceleration of the particle, then a particle at rest would have no weight. Again, this is pure silliness. If there were some observation of this, then maybe it would be interesting, but there hasn't been and this isn't.

$f=mg$ does work for a stationary particle. The force do to gravity of a particle at rest is $mg$ which opposed by some normal force that is equal and opposite and therefore the sum of all forces is 0.

Your "model" is not a model because it does not describe anything real. You are trying to find a way to solve a problem that arises because of a false assumption. The resolution should be obvious. I'm pretty much done with this thread since you appear to be sticking to your guns and uninterested in actual study. Your "model" predicts things that don't happen. Give it up.

Can I say how much I have enjoyed this tread and all the input from contributors.

If as you say I predict things that at this moment in time can not be observed makes my theory wrong so be it. I am not, as I am sure you guess, a professional physicist . I am not trying to create a new model for the scientific community. I am only excising my enquiring mind

You are right I am sure that it is pure silliness to suggest a particle not an object at rest has no weight. I was hoping for a more informative and reasoning other than silliness. Particularly as you can produce no experimental evidence to show me wrong

You keep stating f=mg for a particle is correct you are right it is for an object stationary or moving, and a moving particle, but not a stationary particle as I have shown. So there are no challenges to mechanics there never was I always said the effect was probably not observable.

I am still not sure if the effect can be observed and measured that is for better physicists than me to work out. Also my conclusion is only F=MA which seems highly satisfactory.

Please before you go could you explain my false assumption. I have only used conservation of energy, Heisenberg's Energy-Time uncertainty principle and 2nd law of thermodynamics. oh and causality.

Even if you cannot do this as you perceive me as a waste of space and do not wish to waste your time I really do thank you and the others for your efforts and time indulging my enquiring mind. I have learned a great deal

If that sounds sarcastic it really is not. It is entirely genuine and I wish you all well.

 

[Dale]

My model does absolutely describe the motion of a general moving particle.

No, it doesn't. Your model is incomplete meaning that it is underdetermined. While the correct solution is compatible with your model there are also incorrect solutions which are compatible with your model. For example:
$$x(t)=y(t)=\frac{E}{gm} \left( 1- e^{-\sqrt{2}gt} \right)$$
This is not a correct solution, but it satisfies your model by maintaining a constant total energy with KE decreasing as the object moves to an area of higher PE.

Your model is WRONG, even for moving particles. It does not have enough information to select the correct trajectory which is compatible with energy considerations. For that, you need the principle of least action.

However I conclude that gravitaional force is not equal to mg but ma where a is the actual acceleration of the actual particle not a theoretical value.

To be precise f=mg does not work for a stationary particle but does at all other times.

This conclusion is also WRONG. Please work on learning actual mainstream physics rather than trying to invent new models. The current models have been well tested for centuries, and they work. There is good reason to learn them very well before attempting to make any improvements. Your proposed model is NOT an improvement, it is a mistake.

 

[sophiecentaur]

I don't think he's switched to 'receive' on this one.

 

[Adeste]

No, it doesn't. Your model is incomplete meaning that it is underdetermined. While the correct solution is compatible with your model there are also incorrect solutions which are compatible with your model. For example:
$$x(t)=y(t)=\frac{E}{gm} \left( 1- e^{-\sqrt{2}gt} \right)$$
This is not a correct solution, but it satisfies your model by maintaining a constant total energy with KE decreasing as the object moves to an area of higher PE.

Your model is WRONG, even for moving particles. It does not have enough information to select the correct trajectory which is compatible with energy considerations. For that, you need the principle of least action.

This conclusion is also WRONG. Please work on learning actual mainstream physics rather than trying to invent new models. The current models have been well tested for centuries, and they work. There is good reason to learn them very well before attempting to make any improvements. Your proposed model is NOT an improvement, it is a mistake.

Hi my model is not wrong it needs no information on moving or stationary objects as it makes no difference to the accepted model except when a particle is not moving. It makes no new prediction for moving particles it does not interfere in any way with excepted observable models.

It only says that when a particle not an object is stationary it will not experience a force. As I am lead to believe this is never observable therefore this cannot I believe be shown to be wrong. Clearly you could say it was irrelevant and you may be correct time will tell.

You are correct the present models have been rigorously tested and work I have said this before. Theories on the present testable/observable world are not changed in my model in any way stationary or moving.

I am not trying to improve the models science use it is a personal model of the world to aid my understanding. I thank you for help in refining my model and sorting out in my mind the problem of action at a distance which always seemed nonsense.

I really thank you for your time and patience while I indulged my enquiring mind.

I wish you well

 

[Dale]

Hi my model is not wrong it needs no information on moving or stationary objects as it makes no difference to the accepted model except when a particle is not moving. It makes no new prediction for moving particles it does not interfere in any way with excepted observable models.

Yes, your model as you have presented here is wrong for moving objects. I mathematically proved it by showing a trajectory which satisfies your model but is not correct. Your model differs significantly and does make new predictions compared to correct models, as shown above.

If you disagree and believe that your model is correct, then you need to demonstrate in which way my posted trajectory violates your model. If my posted trajectory does not violate your model then your model is wrong, as I have claimed.

 

[Adeste]

I don't think he's switched to 'receive' on this one.

As I am sure you have noticed I have finished my enquires my model is complete. Can I thank you also for your thoughts and time.

I wish you well I really enjoyed it

ps great signature

 

[Adeste]

Yes, your model as you have presented here is wrong for moving objects. I mathematically proved it by showing a trajectory which satisfies your model but is not correct.

My model does not predict any trajectory sorry if I presented it in a way such that you think it did I apologise. I specifically said it does not apply to moving particles.

Again thanks

 

[Dale]

I specifically said it does not apply to moving particles.

I am calling "BS" on this:

as a body moves to an area of lower gravitational it loses gravitational potential energy which is converted into KE and hence it moves faster hence accelerates

Care to revise your statement?

The whole point of this thread is that you believe that the above model correctly describes the physics of a moving body. You were then concerned by the fact that this model admits a non-physical solution for a stationary body, namely the solution where it doesn't move at all.

I demonstrated that it also admits a non-physical solution for a moving body. Therefore, since it admits non-physical solutions for moving bodies it should come as no surprise that it admits non-physical solutions for stationary bodies also.

There is no difference wrt moving and stationary bodies, the model fails for both.

 

[DrewD]

My model does not predict any trajectory sorry if I presented it in a way such that you think it did I apologise. I specifically said it does not apply to moving particles.

Again thanks

Haha wait! what? you have a model that doesn't predict things? A stationary particle has a trajectory. It is one with momentum equal to zero.

I know I said I wouldn't come back, but this is starting become funny.
Adeste's "model" predicts things that are counter to current theory, BUT since his/her model only applies to particles that are defined by the fact that they follow this model, there is no problem with the fact that current theory predicts different things. So we have a theory that ONLY applies to stationary particles that are not effected by Newton's laws and the theory's conclusion is that these particles are not effected by Newton's laws! Inevitably, they will also not be effected by QM because those are different particles for which this theory doesn't apply.

I think this might actually turn out to be a good example to use in HS physics to show why energy conservation is not sufficient to describe reality and what the difference between science and pseudoscience is.

 

[IsometricPion]

If Δx= 0 then Δpe =0 ∴Δke =0 ∴Δv=0 ∴acceleration = 0

I think this is the key line of reasoning in your model. Δx = 0 → Δpe = 0 holds due to the assumption that potential energy is only a function of x (which holds in the case of a uniform gravitational field). Δpe = 0 → Δke = 0 holds by definition of the total energy and the assumption of its conservation (this statement also holds true in the above discussion). Δke = 0 → Δv = 0 holds due to the assumption that kinetic energy is only a function of velocity (which is true in the above discussion also). Δv = 0 → acceleration = 0 holds if acceleration is a function only of velocity (or change in velocity over a time period of constant length). This last implication does not hold in a uniform gravitational field.

The instantaneous acceleration of an object does not depend on the instantaneous velocity of the object (for uniform gravitational fields). Therefore it is not possible to obtain from the velocity of an object (at one point in time) any information about its acceleration (for uniform gravitational fields).