recklessrog....in your demo.....I would not describe the tube heat coming from gravity. I would say that it comes from the magnet's kinetic energy. Like you say.....reverse the direction...or use a horizontal direction....and you get the same effect....because it does not depend on gravity....just depends on magnet kinetic energy.

Drop a free fall and a tube fall. Measure the kinetic energy at the tube exit height on both falls. They will have different exit times. That difference in kinetic energy is the tube heat.

It does not matter where the initial potential comes from.......for where ever it comes.....it must be converted to KE for tube heat. Whether the magnet has KE or the tube has KE........KE is the form of energy needed for transfer.

In this setup you are using gravity as the prime mover. But whatever prime mover you use.....you must convert to magnet KE.

Just like a generator......magnetic KE(flux) is converted to current. A linear generator.

 

Well, that is a more detailed description of what happens actually. But while describing simple phenomena with Newtonian physics that "storing of gravitational potential energy" is more commonly used. For example, we use Newtonian gravity instead of Einstein's curved space-time when we describe the dropping down of an apple, although curved space-time is more true and rightly explained.

Hi,

I have to agree. We can not use old outdated theories like "Newtons Universal Theory of Gravitation" to solve a problem that involves the possible corpuscular nature of gravity or the lack of it.
According to Einstein, gravity is not a force but a property of space time. We treat is as a force, but that's a farce. So the force is a farce ;-)
This also does away with the notion of a gravitational field, but i dont think anyone is 100 percent sure if gravity can be quantized or not. So the field is a farce also

If i was going to calculate something like the movement of some planets and stars i would probably use all those old theories, but that doesnt mean that we can solve more explicit problems that way. We need more research.

 

It is good to know that the question the young lad put to me has resulted in all the interest and varied responses, also I'm pleased that he thinks "outside the box" as all too many accept the given statements without question. With more like him, the future looks bright!
What I will do at the next session, is present these replies to the group for them to digest as I'm sure their young brains can assimilate the information quicker than me.
Thank you everyone for your input, and anymore are of course very welcome
P.S someone did think the thread was being hijacked, but I didn't think that at all, divergent maybe, but that brings more scope into the discussion.

 

We need more research.

Oh good grief. No wonder business people hate engineers!
(I've been both, so tongue in cheek.)

No additional research is needed on Newton's Laws of motion and thermodynamics in order to answer this kid's simple question. Challenging or expanding those well established laws may be a valid area of study, but a Sheldon Cooper answer in not needed to answer this Penny question with precision far beyond what could detected in any reasonable experiment.

I suspect the key insight the kid was lacking was the work done by the demonstrator in lifting the magnet. Once you complete the energy balance, there is no mystery here.

 

The discussion centers around trying to figure out if gravity can be "used up" somehow, and what i think is a better discussion for this is to think of systems that are relatively independent of each other. One way to do this is to think of a space ship traveling in a straight line through space until it comes close to a sizable planet.
When the space ship gets close, the gravitational field pulls on the ship and thus the speed increases. It gets a "free ride" for a while. Once the ship gets just past the planets center, it turns toward the planet slightly and continues on a curve in such a way that it does not get pulled in but flies off into empty space again, but now going faster.

The question now is that the ship started out at a certain speed and with no extra energy of it's own, but is now going faster, even though in a different direction, so where did that extra energy come from. It must have come from the "gravitational field" of the planet. So the obvious question now is, what was taken from that planet that supplied the energy to speed up the ship.

What you're describing is called a gravity assist. The key part that you're missing is the notion of reference frames. From the perspective of the planet (that the ship is using as a slingshot), the ship gains zero net speed after performing the maneuver: it speeds up on approach, and slows down an equal amount as it departs. But from the perspective of, say, the Sun -- which the planet is orbiting -- if the ship's departure path is in the direction of the planet's orbit, then the ship has gained speed.

Specifically, the ship's extra speed (relative to the sun) represents an increase in its is linear momentum. Because momentum is conserved, the planet's linear momentum decreases by an equal amount, but since the planet's mass is so very large, the effect on its orbital velocity is negligible.

Wikipedia has a good article on the subject:
https://en.wikipedia.org/wiki/Gravity_assist

 

Oh good grief. No wonder business people hate engineers!
(I've been both, so tongue in cheek.)

No additional research is needed on Newton's Laws of motion and thermodynamics in order to answer this kid's simple question. Challenging or expanding those well established laws may be a valid area of study, but a Sheldon Cooper answer in not needed to answer this Penny question with precision far beyond what could detected in any reasonable experiment.

I suspect the key insight the kid was lacking was the work done by the demonstrator in lifting the magnet. Once you complete the energy balance, there is no mystery here.

Hello there,

Simple is as simple does. If you use a simple equation, you get a simple answer. Unfortunately, that does not mean you get the right answer. The answer you get is commensurate with the equation you use.
You got the answer YOU wanted, thus you call it valid. The question is, did you look deep enough or accurately enough.

 

What you're describing is called a gravity assist. The key part that you're missing is the notion of reference frames. From the perspective of the planet (that the ship is using as a slingshot), the ship gains zero net speed after performing the maneuver: it speeds up on approach, and slows down an equal amount as it departs. But from the perspective of, say, the Sun -- which the planet is orbiting -- if the ship's departure path is in the direction of the planet's orbit, then the ship has gained speed.

Specifically, the ship's extra speed (relative to the sun) represents an increase in its is linear momentum. Because momentum is conserved, the planet's linear momentum decreases by an equal amount, but since the planet's mass is so very large, the effect on its orbital velocity is negligible.

Wikipedia has a good article on the subject:
https://en.wikipedia.org/wiki/Gravity_assist

Hello there,

I am going to wait *approximately* one hundred years to read all of that article. I'll try not to make that too *significant* though.

Approximate and significant can be stumbling blocks that get in the way of finding out the truth in a matter. If we allow these to get in the way i think we can miss some interesting things. YES, it is certainly a necessary evil that we must use in the practical world so that we can arrive at answers that are timely, but in the thorough investigation of the universe, we usually need better.

If you can calculate the position of the ship and planet and Sun to within about 1e-35 meters for the entire trip, then i think you can prove that nothing has changed permanently.

It's simple to state that everything 'bounces back'. It is very hard to prove it though. When we are dealing with a problem that involves gravity and we really want to know what is happening, then we have to understand gravity better than anyone on earth knows up to this point in time.

 

I could forgive thus phrasing because the quote marks are implying something - not clear but I won't push it. But this next sentence is the dumbest thing I have ever heard... The capitalization takes away all question of an implied meaning of work origin - you flat-out claim that gravity does work!?!?
Work = force x distance
Force = mass x acceleration
Therefore,
Work = mass x acceleration x distance.

Since gravity is an acceleration, claiming gravity is doing work ("work was done BY gravity") is comperavle to saying something like Distance is done BY Velocity. Ignorant.

I'm going to with hold deciding this whole question until TCM and JJ give their thoughts on it. They are the total deciders in most things.

 

Newton said that an object in motion will stay in motion unless acted upon by another force. In other words, an object moving through space will continue to do so endlessly - or until another force interacts with it. The same can be said of an object at rest. It will remain at rest unless acted upon by another force. In this case the object is a magnet. At rest it is doing nothing. It may be sitting in the grass or on a students desk - wherever. It's doing no work. When one picks up the magnet he is acting on that object that was (I said WAS) at rest and imparting some force to it. Force to raise it to a given height, then dropped. Eventually the magnet will fall back to earth because the force of gravity was active upon it. But when it came to rest on the desk the force of the desk supporting it against the pull of gravity will cancel out the gravity and the magnet will lay motionless on the desk. The work being done is in lifting it, then releasing it. The next force to act upon that object was gravity, to accelerate it toward the surface of the desk. Having fallen through the copper tube the magnet is met by yet another force, that of the generation of electromechanical energy, forcing the magnet to resist the pull of gravity. As the magnetic field slows down the progression of the magnet the magnet slows down, thus creating less electromagnetic energy. The magnet now falls at a greater speed (just follow me for a second). Accelerating again creates a greater resistance and thus slows the magnet down again. So in reality the magnet's fall and the electromotive force cancel each other out at some given rate of travel. The magnet falls slow through the tube because of the interaction of multiple forms of force - gravity - mass - electromotive resistance to name a few.

The original question posed by the student was "Is gravity consumed?" Unlike using fuel to move an object to a higher potential, gravity is omni-presnet around all objects of mass. The greater the mass the greater the gravity. You MIGHT tell your student (and rightfully so) that when you lift an object off the ground that object moves. But so does the earth (ever ever ever so slightly). And when you drop that object the earth again moves ever so slightly toward the object because both have mass. The two are attracted together. So is gravity consumed? How would one convince the student the gravity is not consumed becomes just as valid a question. So we go back to the spring analogy. You stretch a spring, you impart energy. You release the spring, it collapses and the energy you put into it is consumed. Noting that it's the energy you PUT INTO it. The spring is not consumed (for the sake of argument lets assume it never wears out or loses tension).

The answer to the student's question is "No, gravity is not consumed because it is not a source of fuel. It is a result of mass. Otherwise, if gravity was consumed then eventually all atoms would consume their energy and their electrons would fly off into oblivion or collapse into their protons and merge with them to become neutrons.

And that's my uneducated opinion on the subject.

By the way Ron, didn't mean to sound disrespectful if I came off that way. You're a teacher, you certainly know more than I do. Or at least that's my assumption.

 

Newton said that an object in motion will stay in motion unless acted upon by another force. In other words, an object moving through space will continue to do so endlessly - or until another force interacts with it. The same can be said of an object at rest. It will remain at rest unless acted upon by another force. In this case the object is a magnet. At rest it is doing nothing. It may be sitting in the grass or on a students desk - wherever. It's doing no work. When one picks up the magnet he is acting on that object that was (I said WAS) at rest and imparting some force to it. Force to raise it to a given height, then dropped.

In general, yes. But just like when the magnet is sitting on the desk no net work is being done on it, so two when it is being lifted up at constant speed no net work is being done it. The person doing the lifting is doing work. That work is being done on magnet. So far, so good. But that is not the only force acting on the magnet -- so if gravity. The net force on the magnet, if moving upward at constant speed, must be zero. No net force, no work. But the person is doing work on the magnet. What gives? Simple. That magnet is doing work on whatever is providing the other force -- namely gravity. So the net effect is that when you lift the magnet, you are doing work on the gravitational field and storing energy in it.

Imagine an object sitting on a spring. You now push down on the object and compress it at constant speed. You are doing work on the object, but object is, in turn, doing exactly the same amount of work on the spring. So no net work is done on the object and, thus, the energy stored in the object has not changed. Once the spring is compressed (and perhaps clamped in place waiting a trigger to release it) the work energy you expended is not stored in the object, but in the spring. That same with lifting the magnet.

 

Wasn't aware I said the magnet was in possession of stored energy. Semantics I guess.

In general, yes. But just like when the magnet is sitting on the desk no net work is being done on it, so two when it is being lifted up at constant speed no net work is being done it. The person doing the lifting is doing work. That work is being done on magnet. So far, so good. But that is not the only force acting on the magnet -- so if gravity.

I agree with you on the concept of a magnet being lifted at a constant rate. The energy being put into lifting the magnet is met by the counter energy of gravity. Gravity being the weaker form of energy, the person lifting the magnet is doing the work - as you said, not the magnet. The magnet is like that object in space either at rest or in motion. Another force is being applied, this time by the teacher. When the magnet is released the force of gravity accelerates the magnet downward. Again the magnet is doing no work. But unlike the work the teacher did in lifting the magnet, gravity is not doing any work per se'. Gravity is pulling down on the magnet in opposition to the work the teacher initially did in lifting the magnet. When the teacher lifted the magnet he consumed energy (breakfast in this case). (or lunch if after lunch)

But the person is doing work on the magnet. What gives? Simple. That magnet is doing work on whatever is providing the other force -- namely gravity.

Not sure I follow you on this. Yes, the teacher is doing the work. I disagree (respectfully) that the magnet is doing any work. It's mass is being pulled down by gravity but it's not doing any work. For all intents and purposes, the teachers hand is the same as the desk. Though the hand is in motion, the magnet is still at rest, relatively speaking.

So no net work is done on the object and, thus, the energy stored in the object has not changed.

Agreed. I don't think I was trying to say that an object had stored energy. A spring may have stored energy, gravity may be a source of stored energy (so to speak), a moving object may have stored energy, but an object being acted upon by an external force - at a given rate - does not have any energy. So when the teacher lifts the magnet at a given rate of travel, once the initial acceleration has occurred, there is no more energy imparted to the magnet. However, when the lift terminates, there is a release of energy because the magnet does not continue upwards. Gravity has acted as an external force upon the magnet to bring it to a neutral rest in the palm of the hand of the teacher.

Once the spring is compressed (and perhaps clamped in place waiting a trigger to release it) the work energy you expended is not stored in the object, but in the spring. That same with lifting the magnet.

Again I agree with you. The potential is in the spring. In the case of the magnet the potential energy is in the gravity and the elevation. And not the magnet.

So are we disagreeing with each other? I don't see it that way. And as I said as well, this is my uneducated opinion. Meaning I'll yield to greater minds and knowledge anytime someone knows more on the subject than I do. And that's pretty easy. I hope any disagreement we may have is not over semantics.

 

Wasn't aware I said the magnet was in possession of stored energy. Semantics I guess.

You didn't, but it seemed to be implied. Whether it was or not, intentionally or not, it's pretty clear from your follow-up that you aren't claiming that. So we're good on that point.

I agree with you on the concept of a magnet being lifted at a constant rate. The energy being put into lifting the magnet is met by the counter energy of gravity. Gravity being the weaker form of energy, the person lifting the magnet is doing the work - as you said, not the magnet. The magnet is like that object in space either at rest or in motion. Another force is being applied, this time by the teacher. When the magnet is released the force of gravity accelerates the magnet downward. Again the magnet is doing no work. But unlike the work the teacher did in lifting the magnet, gravity is not doing any work per se'. Gravity is pulling down on the magnet in opposition to the work the teacher initially did in lifting the magnet. When the teacher lifted the magnet he consumed energy (breakfast in this case). (or lunch if after lunch)

We are in bulk agreement on most things, but there are some devils in the details.

The notion of "work" is a very precise one in physics. It is NOT the notion we think of every day when we think of someone doing work digging a garden or lifting up a magnet.

The term "work" in physics is the dot product of a Force vector and a displacement vector -- which equates to a transfer of energy due to a force acting through a distance.

If A is exerting a force on B, then because there are always equal and opposite forces, B is also exerting a force on A. If B is moving in the direction of the force exerted on it by A, then A is doing positive work on B and so energy is being transferred to B. At the same time, B is doing negative work on A, so energy is being extracted from A. Work is always the energy transferred TO the object that the work is done ON (and, since work can be negative, energy can actually be being extracted).

So when the magnet is released, the ONLY force acting on it (ignoring air resistance and assuming no metal tube anywhere around -- it's just an object falling under the influence of gravity) is gravity, which is acting in the direction of motion. Thus the work done ON the magnet is positive and it is gaining energy. That energy can't be coming from the teacher, who is no longer interacting with the magnet. The energy is coming from whatever is applying the force on the magnet and that is gravity. The fact that the energy originally came from the teacher is immaterial, as is the fact that it originally came from their breakfast which originally came from a cow which originally came from some hay which originally came from the sun which originally came from the Big Bang. All the matters is where the energy is coming from that is being transferred into the magnet as the work is being done on it. The ONLY place that can come from is the gravitational field.

As the teacher moves the magnet up, they are doing work on the magnet because the teacher is exerting a force on it in the direction it is moving. No one is saying otherwise. But the gravitational field is still exerting a force on the magnet in the direction opposite the motion, and thus work is being done ON the gravitational field BY the magnet. Therefore energy is being transferred from the magnet to the gravitational field. It is no different than if I place a cutting board on top of a potato and then push down on it. The force on the top of the potato is coming from the board, not me. The board is acting as a transfer agent and the force I apply to it is immediately transferred to the potato, but the potato doesn't care -- it doesn't know that I exist. It knows only that a cutting board is exerting a force on it. The same with the magnet. As I lift it I transfer energy into it, but that energy is immediately transferred into the gravitational field.

Agreed. I don't think I was trying to say that an object had stored energy. A spring may have stored energy, gravity may be a source of stored energy (so to speak), a moving object may have stored energy, but an object being acted upon by an external force - at a given rate - does not have any energy. So when the teacher lifts the magnet at a given rate of travel, once the initial acceleration has occurred, there is no more energy imparted to the magnet. However, when the lift terminates, there is a release of energy because the magnet does not continue upwards. Gravity has acted as an external force upon the magnet to bring it to a neutral rest in the palm of the hand of the teacher.

As I described before, when the lifting of the magnet first starts, there is an imbalanced force and so the teacher does more work on the magnet than the magnet does on the gravitational field. The difference IS in the magnet in the form of kinetic energy. At the end of travel, the opposite is true. The teacher is doing less work on the magnet than the magnet is doing on the gravitational field, and so energy is extracted from the magnet and that is manifested by the loss of kinetic energy. These exactly balance out and we can make both as small as we want by just moving the magnet more slowly. These have NO effect on the end result.

So are we disagreeing with each other? I don't see it that way. And as I said as well, this is my uneducated opinion. Meaning I'll yield to greater minds and knowledge anytime someone knows more on the subject than I do. And that's pretty easy. I hope any disagreement we may have is not over semantics.

Again, I think the disagreements are just on some of the details, but those details are important to really understanding what is going on.

 

Oh good grief. No wonder business people hate engineers!
(I've been both, so tongue in cheek.)

No additional research is needed on Newton's Laws of motion and thermodynamics in order to answer this kid's simple question. Challenging or expanding those well established laws may be a valid area of study, but a Sheldon Cooper answer in not needed to answer this Penny question with precision far beyond what could detected in any reasonable experiment.

I suspect the key insight the kid was lacking was the work done by the demonstrator in lifting the magnet. Once you complete the energy balance, there is no mystery here.

 

If you can calculate the position of the ship and planet and Sun to within about 1e-35 meters for the entire trip, then i think you can prove that nothing has changed permanently.

It's simple to state that everything 'bounces back'. It is very hard to prove it though. When we are dealing with a problem that involves gravity and we really want to know what is happening, then we have to understand gravity better than anyone on earth knows up to this point in time.

The fundamental principle involved in a gravity assist is the conservation of momentum, F = dp/dt = 0. Gravity's role is to provide the F, but there's nothing fundamentally privileged or special about this choice of F (other than its convenience at stellar scales); we could just as well perform a "gravity" assist on rotating electric charges with the electric field.

There's no need to appeal to (or even understand) Planck-scale physics to explain the speed-up.

 

A big thank you to everyone who replied to this thread from the members of my local club.
I printed out copies and the group is now actively discussing the subject in detail based upon the information received.
We have 30 members, 14 of which regularly attend the meetings. Ages range from 10 to 87!
We also sometimes fire up my radio shack for additional fun.
It's good to see the hobby is still so alive and attracting new people old and new

 

Glad to hear it. Sounds like a good group and they are interested in trying to really understand thing.