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-   -   KE of system / different reference frames question (http://www.physicsforums.com/showthread.php?t=564246)

jduffy77 Jan1-12 02:44 PM

KE of system / different reference frames question
 
I am re-posting this question here in a new thread as Humber mistakenly posted it in a two year old thread.

Quote:

Quote by Humber (Post 3691563)
This post appeared on a ddwfttw forum:

Quote:

Quote by uncool
For those who actually care, relative to any frame other than that of the ground, the ground does have energy. It is possible to get energy from the ground - in fact, this is exactly what happens, for example, in KERS. The non-ground frame analysis would go as follows:

Say we have a 10 kg object initially moving at 10 m/s relative to the ground. We choose to start analyzing in the frame where this object is initially at rest - so the ground is initially moving at -10 m/s.

We use KERS to brake the object at a rate of -1 m/s^2, or equivalently, a force of -10 N. By Newton's third law, that means that there is a force of 10 N on the Earth.

As the object is at rest relative to this frame, there is no relevant kinetic power on the object itself. However, there is kinetic power on the Earth; using P = F*v, we get that the Earth is losing kinetic energy at a rate of -100 W, so by conservation of energy, the KERS can be storing energy at a rate of 100 W.

Note that this is exactly the same as what we get in the frame of the ground, as there, F = -10 N, v = 10 m/s, so the object is losing kinetic energy at a rate of -100 W, so by conservation of energy, the KERS can be storing energy at a rate of 100 W.


The above idea, seems to be quite commonly accepted amongst those claiming to have physics degrees, but it seems to me to be a the result of literal thinking, and a rather confused idea of what frames of reference means.

The energy for the KERS, initially comes from the car's fuel, some of which ultimately ends up as kinetic energy of the car. It is that energy which is recovered by KERS, and does not come from the ground.

I am interested in hearing options for or against either claim.


Ken G Jan1-12 03:55 PM

Re: KE of system / different reference frames question
 
I'm not sure what your issue is with "literal thinking", to me the quote is a clear example of not thinking literally. Literal thinkers imagine that energy is innate, so a car that is moving along the road has it, in any frame of reference. But energy is frame dependent-- conservation of energy means the energy stays the same in a given frame, it doesn't mean it is the same in all frames. So choosing different frames always repartitions the energy, and therefore it also changes the language about what the energy is doing or where it is coming from. This is all perfectly normal, it is what you find when you relax the need to think literally.

So there is no problem at all with saying that a car can extract energy from the road when it uses KERS, but the language sounds awkward unless one is being extremely clear about the choice of reference frame. For example, one can have a car that accelerates from rest to 10 m/s, and if we start in a frame already moving at 10 m/s, this will look like a car that uses its fuel energy yet loses all its kinetic energy-- the missing energy shows up in the Earth's kinetic energy in that frame. That's just how it is, in that frame. We can argue that isn't the best frame to think about, or we can argue it is, but it certainly isn't wrong.

256bits Jan1-12 04:15 PM

Re: KE of system / different reference frames question
 
I had to look up KERS and that is Kinetic Energy Recovery System.

It is so far so good up to the 5th paragraph.
The 10kg object can be considered to be at rest in a frame, call it frame A, moving at 10 m/s relative to the earth. But once the braking occurs, the object will have a force applied to it and begin to move relative to that frame. ( Otherwise you are dealing with a moving frame of reference. ) At the end of it all, the object will have a velocity of -10 m/s in frame A.

Suppose there are 2 cars moving side by side. You have a tiny window to look out from car 1 out but all you can see is the car 2 beside you, which would appear to be not moving relative to your reference frame A. Person X on the earth can see both Car1 and 2 and conclude that both cars are mving at 10 m/2 relative to the earth.
Car2 signals you that he is going to apply KERS. As he does so, to you in Car 1, it would look as if Car2 is accelerating in the backward direction up to a maximun of -10 m/s. You in Car1 would begin to wonder how in the world did Car2 accelerate if Car2 was using KERS.

256bits Jan1-12 04:48 PM

Re: KE of system / different reference frames question
 
Quote:

The energy for the KERS, initially comes from the car's fuel, some of which ultimately ends up as kinetic energy of the car. It is that energy which is recovered by KERS, and does not come from the ground.
A really bad statement all in all. It is not put together in any coherent fashion. You usually find something like this on a site promoting the newest crackpot idea which is meant to confuse, and not for an explanation of KERS. I cannot tell if the writer is saying that the energy of the fuel is being recovered or the kinetic energy of the vehicle. If he is saying it is the energy of the fuel, then why not go farther back and say it is the energy of the sun trapped in the plant material, from 600 million years ago, that became hydrocarbons through transformation under the earth's surface.

The fuel provides energy to the car to move. The chemical energy is transformed into kinetic energy. KERS would recover some of the kinetic energy.

rcgldr Jan1-12 04:55 PM

Re: KE of system / different reference frames question
 
I think the point missed in the old DDWFTTW thread is the source of the force at the tires. For a KERS vehicle, the force at the tires equals the mass of the vehicle times it's rate of acceleration (or deceleration) (ignoring rolling resistance and aerodynamic drag). For a DDWFTTW vehicle, the force at the tires equals the torque used to drive the propeller divided by the radius of the tires, which is independent of the mass of the vehicle.

mender Jan1-12 05:25 PM

Re: KE of system / different reference frames question
 
Quote:

Quote by 256bits (Post 3691710)
Otherwise you are dealing with a moving frame of reference.

Not a problem since for any given situation, there is only one non-moving frame of reference and an infinite number of moving ones.

I think what you're meaning is an accelerating frame of reference.

Humber Jan1-12 05:30 PM

Re: KE of system / different reference frames question
 
Quote:

Quote by Ken G (Post 3691687)
I'm not sure what your issue is with "literal thinking", to me the quote is a clear example of not thinking literally. Literal thinkers imagine that energy is innate, so a car that is moving along the road has it, in any frame of reference. But energy is frame dependent-- conservation of energy means the energy stays the same in a given frame, it doesn't mean it is the same in all frames. So choosing different frames always repartitions the energy, and therefore it also changes the language about what the energy is doing or where it is coming from. This is all perfectly normal, it is what you find when you relax the need to think literally.

So there is no problem at all with saying that a car can extract energy from the road when it uses KERS, but the language sounds awkward unless one is being extremely clear about the choice of reference frame. For example, one can have a car that accelerates from rest to 10 m/s, and if we start in a frame already moving at 10 m/s, this will look like a car that uses its fuel energy yet loses all its kinetic energy-- the missing energy shows up in the Earth's kinetic energy in that frame. That's just how it is, in that frame. We can argue that isn't the best frame to think about, or we can argue it is, but it certainly isn't wrong.

That is indeed the thinking I was taking about. That "all things are relative".

Acceleration is not one of those things, and the fuel in that tank, is another.
Can you justify that the fuel consumed, is capable of accelerating the Earth by same amount it accelerates the car?

Humber Jan1-12 05:35 PM

Re: KE of system / different reference frames question
 
Quote:

Quote by 256bits (Post 3691735)
A really bad statement all in all. It is not put together in any coherent fashion. You usually find something like this on a site promoting the newest crackpot idea which is meant to confuse, and not for an explanation of KERS. I cannot tell if the writer is saying that the energy of the fuel is being recovered or the kinetic energy of the vehicle. If he is saying it is the energy of the fuel, then why not go farther back and say it is the energy of the sun trapped in the plant material, from 600 million years ago, that became hydrocarbons through transformation under the earth's surface.

The fuel provides energy to the car to move. The chemical energy is transformed into kinetic energy. KERS would recover some of the kinetic energy.

The first few statements are not mine. The claim there, is that the Earth can transfer 100W to the KERS.
But, regardless of its origins, the fuel is the store of some energy form somewhere. It could equally be a battery. When the car accelerates, some of that energy becomes KE in the car's mass, and that is the energy that is recovered, and not something that comes from the ground.

Humber Jan1-12 05:42 PM

Re: KE of system / different reference frames question
 
Quote:

Quote by rcgldr (Post 3691739)
I think the point missed in the old DDWFTTW thread is the source of the force at the tires. For a KERS vehicle, the force at the tires equals the mass of the vehicle times it's rate of acceleration (or deceleration) (ignoring rolling resistance and aerodynamic drag). For a DDWFTTW vehicle, the force at the tires equals the torque used to drive the propeller divided by the radius of the tires, which is independent of the mass of the vehicle.

Yes. F=ma.
When the car brakes, momentum is transferred from the car, to the ground, dp/dt = F.
For a KERS to work, that same force is present, but the KE of the car is not dissipated as heat in the brake pads and disks, but becomes stored in a mass as angular KE, or by direct conversion to electrical energy and so stored in a battery.
Apart form some work at the tyres and surface deformation, which is a result of F, and energy that can't be recovered, there is no energy from the ground.

jduffy77 Jan1-12 06:14 PM

Re: KE of system / different reference frames question
 
Quote:

Quote by Humber (Post 3691792)
The first few statements are not mine.

No, those are correct. The incorrect one he mentioned was not put forth in a coherent fashion was yours though.

Quote:

Quote by Humber (Post 3691792)
The claim there, is that the Earth can transfer 100W to the KERS.
But, regardless of its origins, the fuel is the store of some energy form somewhere. It could equally be a battery. When the car accelerates, some of that energy becomes KE in the car's mass, and that is the energy that is recovered, and not something that comes from the ground.

As was already pointed out; whether or not the energy comes from the ground depends on the reference frame chosen to analyze.

Subductionzon Jan1-12 06:16 PM

Re: KE of system / different reference frames question
 
Quote:

Quote by Humber (Post 3691784)
That is indeed the thinking I was taking about. That "all things are relative".

Acceleration is not one of those things, and the fuel in that tank, is another.
Can you justify that the fuel consumed, is capable of accelerating the Earth by same amount it accelerates the car?

No one has made that claim Humber. You are trying to use a straw man argument.

Why don't you lay out your claim here that all energy must be calculated relative to the Earth and see how that flies here. Or as you claimed elsewhere, that no energy can come from the ground.

RCP Jan1-12 06:38 PM

Re: KE of system / different reference frames question
 
Quote:

Quote by 256bits (Post 3691710)
Otherwise you are dealing with a moving frame of reference.

Quote:

Quote by mender (Post 3691777)
Not a problem since for any given situation, there is only one non-moving frame of reference and an infinite number of moving ones.

I think what you're meaning is an accelerating frame of reference.

Mender old bean!! You've been missed at TR. Did you ever get around to do that test pulling your cart to wind speed and then releasing it along with some packing popcorn? I would still love to see a video of that.

Also looking forward to 256bits response. And what about a decelerating FoR?

Humber Jan1-12 07:12 PM

Re: KE of system / different reference frames question
 
Quote:

Quote by jduffy77 (Post 3691832)
No, those are correct. The incorrect one he mentioned was not put forth in a coherent fashion was yours though.



As was already pointed out; whether or not the energy comes from the ground depends on the reference frame chosen to analyze.

Which is a contradiction of both my statement, and that of 256bits, which is paraphrasing of mine.
When the car accelerates, momentum is transferred from the ground to the car, and when it brakes, the other way. That is conservation of momentum at work, which leaves you to explain if the ground powers the KERS, why fuel is needed to transfer momentum one way, but not the other. And, if you can possibly manage it, please keep your personal remarks and references to the cart to yourself.

jduffy77 Jan1-12 07:29 PM

Re: KE of system / different reference frames question
 
Quote:

Quote by Humber (Post 3691880)
Which is a contradiction of both my statement, and that of 256bits, which is paraphrasing of mine.
When the car accelerates, momentum is transferred from the ground to the car, and when it brakes, the other way. That is conservation of momentum at work, which leaves you to explain if the ground powers the KERS, why fuel is needed to transfer momentum one way, but not the other. And, if you can possibly manage it, please keep your personal remarks and references to the cart to yourself.

Please check your quotes. The two sentences you have quoted were mine but the body of your post is responding to someone else.

mender Jan1-12 08:51 PM

Re: KE of system / different reference frames question
 
Happy New Year, RP!
Quote:

Quote by RCP (Post 3691846)
Mender old bean!! You've been missed at TR. Did you ever get around to do that test pulling your cart to wind speed and then releasing it along with some packing popcorn? I would still love to see a video of that.

Nothing yet and not likely in the foreseeable future, too much real work to do! I haven't stopped in because I didn't think there was anything left but some stirring of the ashes!

ETA: wow - part 30??!!

DaleSpam Jan1-12 09:43 PM

Re: KE of system / different reference frames question
 
Quote:

Quote by Humber (Post 3691792)
When the car accelerates, some of that energy becomes KE in the car's mass, and that is the energy that is recovered, and not something that comes from the ground.

In some valid frames, when the car accelerates it loses KE, and can even come to rest. When it is at rest there is 0 KE. Furthermore, KE can never be negative. So, how do you propose to recover energy from something with 0 energy and whose energy cannot go negative?

Have you even attempted to calculate the energy in a moving reference frame and see how it works out?

RCP Jan1-12 09:50 PM

Re: KE of system / different reference frames question
 
Quote:

Quote by mender (Post 3691949)
Happy New Year, RP!

Nothing yet and not likely in the foreseeable future, too much real work to do! I haven't stopped in because I didn't think there was anything left but some stirring of the ashes!

ETA: wow - part 30??!!

Best wishes for the new year back atcha mender. Yep, Cartville is still accelerating, and FYI humber has now posted pictures of 3 different carts he made. (Film at 11! :smile:) Remember how positive everyone was he would never make one?

And if you or anyone has time, there's still my question above about the FoRs applicable for deceleration?

rcgldr Jan1-12 10:09 PM

Re: KE of system / different reference frames question
 
Although off topic from the orignal DDWFTTW thread ... ignoring any losses due to aerodynamic drag, rolling resistance, conversion of energy, ... , then considering a car and earth as a closed system, then energy (and momentum) of this closed system is conserved.

KERS - extracts energy from earth + car closed system and adds that energy to a device that stores energy (kinetic or potential).

Usage of fuel - extracts chemical potential energy from fuel and adds that energy to the kinetic energy of earth + car closed system.

For both of these cases, the amount of energy converted is independent of the frame of reference, as long as the frame of reference is inertial.

Getting back on topic from the original DDWFTTW thread, someone was wondering if a DDWFTTW vehicle was somehow extracting it's own kinetic energy (wrt ground). The responses that followed tried to explained that a DDWFTTW vehicle extracts energy by slowing down air (wrt ground), and that a DDWFTTW vehicle requires a true wind (wrt ground) in order to operate.


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