Gravity

[CPL.Luke]

when your muscles do work they release a bit of heat into the surrounding enviroment

 

so the glass would warm up minutely (enough to be considered neglagible)

 

lets take an example of me carrying a marble up to the top of a cliff. why would potential energy only be built up while I am carrying the ball up the cliff. if I were to stand there it should continue to build up if what your saying is true because I am still under the influence of gravity. Its still pulling down on me with the same force. so why would potential energy just stop building up when I reached the top of the cliff.

 

or what would happen if an asteroid jsut streaked in from interstellar space as it enters the earths gravitational field it has never been lifted from. I think it would be accelerated towards the earth. however if potential energy was real and a prerequisit for an object to fall, then the asteroid would continue without being affected by the earths gravitational field.

 

 

an object 150 miles above the surface of the ground has less potential energy than an object 100 miles above the ground. know how I came up with that, gravity at 100 miles above the surface of the ground comes under a slightly greater gravitational force than an object located 150 miles above the ground.

[Ophiolite]

why would potential energy only be built up while I am carrying the ball up the cliff. if I were to stand there it should continue to build up if what your saying is true because I am still under the influence of gravity. Its still pulling down on me with the same force. so why would potential energy just stop building up when I reached the top of the cliff.

Set the ball down at the top of the cliff. it sits there. Work is no longer being done. its potential energy does not increase. You have chosen a complex variant of the problem' date=' where you are [i']holding [/i]the ball. it is now unsupported. The work being done by your muscles is not contributing to the potential energy of the ball, but is simply maintaining your hand/arm/all ins its current position.

or what would happen if an asteroid jsut streaked in from interstellar space as it enters the earths gravitational field it has never been lifted from. I think it would be accelerated towards the earth. however if potential energy was real and a prerequisit for an object to fall' date=' then the asteroid would continue without being affected by the earths gravitational field.

[/quote']Think of potential energy as energy of position, if that helps. It doesn't matter (no pun intended) how it got to that position.

[swansont]

when your muscles do work they release a bit of heat into the surrounding enviroment

 

so the glass would warm up minutely (enough to be considered neglagible)

 

The physics definition of work is not the same as the lay definition of work. Let's say the glass of water was on a table' date=' in an elevator. Where's the heat flow?

 

lets take an example of me carrying a marble up to the top of a cliff. why would potential energy only be built up while I am carrying the ball up the cliff. if I were to stand there it should continue to build up if what your saying is true because I am still under the influence of gravity. Its still pulling down on me with the same force. so why would potential energy just stop building up when I reached the top of the cliff.

 

Work is the integral of Force (dot) displacement

 

Once you stop moving, there is no more displacement, so no more work is done. No change in PE.

[JaKiri]

Bear in mind that, effectively, all electromagnetic interactions are results of fields of potential energy. Mentioning E-M isn't likely to help your case.

[CPL.Luke]

well how about this

 

if you had the ability to know anything about an object (except its position). the temperature the amount of gravity its being subjected to its velocity, anything. and, you just didn't know exactly where it is. you could make any prediction you wanted about what the object would do in certain circumstances

 

now if you then learned that the object was suspended 3 meters in the air, on the top of a latter. you would only then be able to define its potential energy.

 

this to me would mean that potential energy does not exist as it is an unmeasurable quantity in the world.

 

an electrical potential is completely different however. as you could measure it and prove it experimentally.

 

the electrical field potential is found from, one electrode having a charge of a coulomb electrons, and the other side lacking a coulomb electrons. the two sides are attracted by the e-m field to eachother (which could be measured in volts (as volts can be considered a measure of kinetic energy (correct me if I'm wrong on this one))).

 

My point is that potential energy as described in the above posts by others. can not exist as it is not a measurably quantity.

 

to sight copenhagenism, if it cannot be seen it cannot exist

[Ophiolite]

The potential energy of the body is not measurable in your example because you have excluded the information necessary to compute that: its position.

 

Let me adapt your opening statement.

 

if you had the ability to know anything about an object (except its temperature). its position, the amount of gravity its being subjected to its velocity, anything. and, you just didn't know exactly how hot it is. you could make any prediction you wanted about what the object would do in certain circumstances

 

What you could not do would be to say how much heat it contained. Using your logic therefore, heat is not a measurable quantity and does not exist. (Though perhaps you were just thinking of Aberdeen in the winter.)

[swansont]

well how about this

 

if you had the ability to know anything about an object (except its position). the temperature the amount of gravity its being subjected to its velocity' date=' anything. and, you just didn't know exactly where it is. you could make any prediction you wanted about what the object would do in certain circumstances

 

now if you then learned that the object was suspended 3 meters in the air, on the top of a latter. you would only then be able to define its potential energy.

 

this to me would mean that potential energy does not exist as it is an unmeasurable quantity in the world.

 

an electrical potential is completely different however. as you could measure it and prove it experimentally.

 

the electrical field potential is found from, one electrode having a charge of a coulomb electrons, and the other side lacking a coulomb electrons. the two sides are attracted by the e-m field to eachother (which could be measured in volts (as volts can be considered a measure of kinetic energy (correct me if I'm wrong on this one))).

 

My point is that potential energy as described in the above posts by others. can not exist as it is not a measurably quantity.

 

to sight copenhagenism, if it cannot be seen it cannot exist[/quote']

 

Calculating gravitational PE is as straightforward as calculating electrostatic PE. And just as easy to demostrate experimentally. It is position dependent, so you need to know positions. To limit yourself to a situation where you don't know position, and conclude that PE doesn't exist as a result, is absurd.

 

Volts is not a measure of KE, it's a measure of electric potential, which is potential energy per unit charge. 1 fundamental charge accelerated across a 1 Volt potential difference will give you 1 eV of KE.

[CPL.Luke]

"Calculating gravitational PE is as straightforward as calculating electrostatic PE. And just as easy to demostrate experimentally. It is position dependent, so you need to know positions. To limit yourself to a situation where you don't know position, and conclude that PE doesn't exist as a result, is absurd."

-swansont

 

however in that case an object placed on an elevation above the ground has less potential energy then an object that is placed on the ground, as the gravity on the ground is greater than the gravity the object above the ground experiences as the distance is greater. thus less of a potential.

 

"The potential energy of the body is not measurable in your example because you have excluded the information necessary to compute that: its position.

 

Let me adapt your opening statement.

 

if you had the ability to know anything about an object (except its temperature). its position, the amount of gravity its being subjected to its velocity, anything. and, you just didn't know exactly how hot it is. you could make any prediction you wanted about what the object would do in certain circumstances"

-ophiolite

 

 

however I allowed you to know the strength of any field it experiences. which according to swansont should be enough to get to know the potential (from fields) that it has. but moreover the example was trying to state that its the same object with the same measurable properties as before. you are enforcing an extrapolated anthrocentric concept onto it. I think anyone would agree with me that potential energy is in no way a measurable concept other than when talking about fields. which I now agree with from swansont's explanation (thank you for it). but you could not measure the potential energy that is derived from lifting an object up to the top of the ladder. it is an enforced concept which doesn't necassarily have to exist

 

from this discussion I was able to realise (among other things) that potential energy is an attempt to make sure the idea of work energy.

 

 

now though just ask the question of the quantum mechanical idea of how the electromagnetic force works.

 

it works through the exchange of photons between two electrons. now this is supposed to (by my limited understanding) reconcile with conservation of energy because they both sent and recieved a photon. but yet their is a repulsionary force observed between the two. so where does the work energy come from? maybe work energy doesn't neccasarily have to exist?

 

now if I am totally wrong with this last example than please say so, because it is entirely possible in my opinion that I think I know more about the quantum mechanical concept of electromagnetism than I really do.

[swansont]

"Calculating gravitational PE is as straightforward as calculating electrostatic PE. And just as easy to demostrate experimentally. It is position dependent' date=' so you need to know positions. To limit yourself to a situation where you don't know position, and conclude that PE doesn't exist as a result, is absurd."

-swansont

 

however in that case an object placed on an elevation above the ground has less potential energy then an object that is placed on the ground, as the gravity on the ground is greater than the gravity the object above the ground experiences as the distance is greater. thus less of a potential. [/quote']

 

The object that is higher has more potential energy; for small separation distances the acceleration due to gravity can be assumed to be constant.

[CPL.Luke]

then lets pretend it is a very tall ladder that goes several hundred miles (or several thousand if need be) up into space, just for the sake of argument

 

 

 

but what about the question of quantum e-m as it seems to be of great pertenance to the topic

 

if to particles repelling or being attracted to eachother is the result of the exchange of 2 photons then where is the work energy in that problem

 

because I would hypothesise that if there is no work energy in that problem then work energy doesn't exist. thus it would not require work energy in any experiment. with no work energy there cannot be any potential energy. as the potential energy in question is essentially stored work energy. (not field potentials)

 

also what about the bending of light in GR because light is massless it does not have any work energy (correct?) yet it still bends. if there is no potential energy present yet it still bends in the presence of gravity then how can potential energy exist

[swansont]

The fact that they exchange photons and exert a force on each other means they are doing work on each other, if there is a net displacement.

 

In GR light bends because the space is warped. PE in GR is a separate issue - mgh or GMm/r are classical formulae.

[CPL.Luke]

right now I think were talking more about the relationship between work energy and potential energy (the kind not involved with fields). if an object is placed on a ladder potential energy ensures that the work energy required to lift the object is conserverd.

 

so our question really comes down to where does the work energy go. I personnally can figure out where it comes from however where it goes is something I need to think about, I personnally will never believe that work energy is conserverd in an unmeasurable quantity in an object. (if you could come up with a way of directly measuring potential energy then I will buy it). I believe in gravitational and electromagnetic potentials, but to me that is a different issue such as in the thought experiment with an object on a ladder, it will be exposed to less of a gravitaional potential because of the fact that there is less of a distance (you may need a really tall ladder to do an actual experiment involving this.

 

however now were talking about the act of lifting an object up a ladder where does the energy required to fight gravity come from?

 

as an example of what Im talking about.

if a chunk of rock is blown off of an asteroid, (gravity here is very weak) now this chunk was by chance sent cerreaning towards the earth. this small chunk which only took a small amount of work energy (stored as potential energy) to release is now falling towards the earth where it would require more potential energy than the object posseses in order to fall. thus violating conservation of energy as more work energy needs to be added to the system.

 

in that thought experiment we discovered that the 1:1 relationship between work and potential energy is not accurate. this means that this model only works for objects that are trying to leave and then returning to the same object (what goes up must come down).

 

now, this proves that the idea of potential energy is a falsehood. so where does the work energy go?

[swansont]

however now were talking about the act of lifting an object up a ladder where does the energy required to fight gravity come from?

 

Immaterial. It only matters that energy is required.

 

as an example of what Im talking about.

if a chunk of rock is blown off of an asteroid' date=' (gravity here is very weak) now this chunk was by chance sent cerreaning towards the earth. this small chunk which only took a small amount of work energy (stored as potential energy) to release is now falling towards the earth where it would require more potential energy than the object posseses in order to fall. thus violating conservation of energy as more work energy needs to be added to the system.

 

in that thought experiment we discovered that the 1:1 relationship between work and potential energy is not accurate. this means that this model only works for objects that are trying to leave and then returning to the same object (what goes up must come down).

 

now, this proves that the idea of potential energy is a falsehood. so where does the work energy go?[/quote']

 

Objects do not require potential energy to fall, the fall because they have potential energy, and that exists because there is a force acting on them. An object far from earth has more potential energy, not less, than an object on the surface. The rock would not have been blown off because of the potential energy it possessed. Energy has to be added, because PE increases with r.

 

Because you made those mistakes, your example proves nothing.

 

The presence of forces dictates that work is done. Potential energy is just a way of representing that, and the math behind it is trivial. You being in denial about the whole thing doesn't change that.

[CPL.Luke]

I didn't say that the asteroid fragment would be released because of its potential energy, I said that it would be released because of some event that occured to release it (an explosion for example, lets pretend someone put a bomb on the surface just so that this event could occur)

 

now what I'm saying is that if you launch an object vertical to the ground, it takes A. energy to accelerate an object to the release speed. B. the energy to deccelerate an object (caused by gravity in this case). conservation of energy dictates that energy cannot be created or destroyed, so when the object is deccelerated because of gravity the work energy that was required to accelerate the object to a certain speed is then depleted as the object's velocity goes to zero. ounce the object is at zero and starts to accelerate back towards the ground ou have to question where the work energy goes (potential energy) then as the object starts accelerating back towards the ground the potential energy is released as work energy and you end up with kinetic energy. in this manner conservation of energy is not violated.

 

however I would say that the objects kinetic energy is just transferred to wherever the field is originating from (in this case the earth). thus conservation of energy is not violated because the energy from the objects launch is conserved in the planet and the remaining kinetic energy of the launched object

 

"Energy has to be added, because PE increases with r."

 

if energy was added that would be a violation of conservation of energy

 

"Objects do not require potential energy to fall, the fall because they have potential energy, and that exists because there is a force acting on them. An object far from earth has more potential energy, not less, than an object on the surface. The rock would not have been blown off because of the potential energy it possessed. Energy has to be added, because PE increases with r."

 

if your trying to say that an object has potential energy because of the force acting on it as in "the fall because they have potential energy, and that exists because there is a force acting on them" then an object far from earth has less of a force acting on it. thus "and that exists because there is a force acting on them" and "An object far from earth has more potential energy, not less, than an object on the surface" are inherent contradictions

 

and lastly

Quote:

Originally Posted by CPL.Luke

however now were talking about the act of lifting an object up a ladder where does the energy required to fight gravity come from?

 

 

 

"Immaterial. It only matters that energy is required."-swansont

end quote

 

 

 

that is a violation of conservation of energy, as energy always comes from somewhere, then energy then has to go somewhere.

[swansont]

if energy was added that would be a violation of conservation of energy

 

 

Take a physics class.

 

I'm done.