Split Thread The validity of classical physics (split from: DWFTTW)

[spacediver]

As an example, consider two lumps of clay on a collision course. When they collide they stick together, conserving momentum but turning some of their KE into heat. As you can easily check, the amount of energy turned into heat is independent of reference frame - in other words if you shift both velocities by the same amount, it doesn't change.

hm now I remember another thing that confused the hell out of me when I was learning this stuff. Perhaps addressing this confusion will help me understand the relative KE "paradox".

Two questions:

Why the hell is momentum conserved when objects lose kinetic energy due to heat? If an object loses kinetic energy, but maintains its mass, then its velocity must decrease. And if it loses its kinetic energy in a manner that doesn't involve the change in velocity or mass of other particles, then total momentum is not conserved.

If I had to guess, I'd say that the answer is that the loss of kinetic energy necessarily involves the change in the mass or velocity of some other particle.

Second question:

Why is the idea of momentum so special? Why not have something called popentum, and make the formula 1/4*m*v*v*v - won't popentum also be conserved? I've always just learned that momentum (p) is mv, but even after browsing through the wiki entry on momentum, I can't seem to understand why mv is more special than, say, m*m*v or m*v*v

I very much appreciate the efforts to help me btw

Happy 2009 to all!

 

[CORed]

Have you ever been in a canoe on a river?

This brings to mind a picture of Humber in a canoe, drifting with the current towards a big rock, trying to use the paddle as a rudder and wondering why the canoe won't turn until it crashes into the rock.

 

[Subduction Zone]

spacediver what makes momentum special is that it is always conserved. The knowledge that it is conserved allows us to use it as a tool in analyzing a system. Kinetic energy can be turned into heat or potential energy, there is no law of conservation of kinetic energy, there is a law of conservation of energy. So again though kinetic energy (mvv/2) isn't conserved the energy part of it is, another useful tool. And as far as your "popentum" idea, if you can find some unique qualities to that figure go ahead, but as far as I know there is no useful knowledge to be found from it.

 

[John Freestone]

Nightroaming hags!

What about a bunch of water molecules? How comes that they can indeed travel at the same speed as water, while other molecules in the water should not? What makes them different? Lighter, heavier? What about molecules with the same weight? And so, what makes a bunch of molecules cobbled together to form a body different from a bunch of molecules cobbled together to form water?

Nice point. I'd also like to reiterate what you observed in relation to humber's reply, i.e. that it was just a dodge. You asked a perfectly sound and potentially enlightening question, which he could not address honestly without leading to his umpteenth failed argument.

As for evidence, it is you who has not yet shown any supportive links or evidence that backs up any of your claims, except for the "evidence" you made up yourself (that is, your drawings). You said many, many times that you will come up with something, but always failed to do so.

Again, I just felt like supporting this point, Chris. Not only does he not "have time" to post anything to support his physics, but he even accuses others who do post links of having to rely on what other people say instead of learning things themselves. He says that the links are misdirection, having demanded them himself. What absolutely disgusting behaviour! What an affront to rational scientific discussion!

Hot air balloons have an engine. A heat engine. Shut up.

Happy New Year, Giant of the Gale-blasts. You're wrong. The "heat engine" of a hot air balloon does not propel it laterally, Curse of the Rain-hall. The example of a hot air balloon was offered in response to your demand for examples of things that travel at the same speed as the fluid medium they are in. Anyone can see that this relates to the lateral movement of a hot air balloon, which is at the same speed as whatever wind is blowing, according to all sources so far found, and certainly of those posted here (score follows shortly - if you don't want to know, look away now).

The heat engine only makes it go upwards, see? Perhaps you can tell us how a hot air balloon moves sideways if there is no wind. Perhaps you'd like to get in one on an absolutely still day (night might be more appropriate, actually, Gnawer of the Moon) and use your heat engine to come and tell me I'm wrong. While you're at it, why don't you pop a souvenir from Scandinavia in your little basket to bring with you?

wikki wikki

The Physics Classroom Tutorial, "Relative Velocity and Riverboat Problems"

Thanks, Michael. I'll call that three-nil. In fact, to hell with it, I remember there was another one earlier, so that's four-nil. That's four links to sites explaining how bodies move in fluids, which conform to classical physics, ZERO links for humber-physics. I note in passing, however, that although I found several exercises on that page from which it could easily be deduced that a body with no motive power of its own would move with the fluid it is immersed in, I did not see that COMPLETELY OBVIOUS FACT spelled out. There is a serious shortage of COMPLETELY OBVIOUS FACTS on the Internet to link to. There should be an enormous website somewhere just dedicated to them: most cats have four legs; the sun is hotter than the earth; oranges are not the only fruit...

A surfer.

I think that's a different issue, roger. I think you surf the moving wave energy, rather than the flowing water. Anyway, it's a different and more complex problem than "pooh sticks", and less well known. Even reasonably intelligent people might not know it. (I imagine you do, in fact, and just mistook the original question for something complicated enough to be worth discussing in the first freaking place.)

 

[sol invictus]

hm now I remember another thing that confused the hell out of me when I was learning this stuff. Perhaps addressing this confusion will help me understand the relative KE "paradox".

Two questions:

Why the hell is momentum conserved when objects lose kinetic energy due to heat? If an object loses kinetic energy, but maintains its mass, then its velocity must decrease. And if it loses its kinetic energy in a manner that doesn't involve the change in velocity or mass of other particles, then total momentum is not conserved.

If I had to guess, I'd say that the answer is that the loss of kinetic energy necessarily involves the change in the mass or velocity of some other particle.

No, that's not correct. Momentum is conserved in that collision because it has no place to go - if all the mass is accounted for, there's nothing to carry it away. Kinetic energy is not conserved, because some gets converted into heat.

The mass cannot change unless the collision is so energetic a nuclear reaction takes place. Lets assume that doesn't happen. Then the state of each object is characterized by its velocity, which is a vector with three components. So we have six numbers initially before the collision (three and three) and six after. Conservation of momentum is an equation that tells us something about (some combinations of) three of those numbers, and conservation of energy tells us about one. That still leaves two free (which in an elastic collision in the center of momentum frame is the direction the two masses fly out in).

In the case of an inelastic collision (where the masses stick), we know there's only three final numbers (the velocity of the stuck together mass), but there's also only three conditions, since energy isn't conserved. So in that case the final velocity is completely determined by conservation of momentum, but it is still conserved.

Why is the idea of momentum so special? Why not have something called popentum, and make the formula 1/4*m*v*v*v - won't popentum also be conserved? I've always just learned that momentum (p) is mv, but even after browsing through the wiki entry on momentum, I can't seem to understand why mv is more special than, say, m*m*v or m*v*v

The quickest answer is, "because". If you take Newton's laws, you'll see that mv is conserved, but mv³ is not.

The deeper answer is that conservation of momentum follows from the invariance of the laws of physics under changes in location, just as conservation of total energy follows from the invariance of the laws of physics with time and conservation of angular momentum from invariance under rotations. It turns out that symmetries like those always lead to conserved quantities - that fact is probably the single most central tool in modern physics. But there are only a few symmetries like that, so there are only a few conserved quantities. And conserved quantities are very useful to label things with.

 

[humber]

I wish they taught this more explicitly in high school. It's a bit of a conceptual leap for me to get my head around this.

We're always taught that energy cannot be created or destroyed, and that makes us think of energy as a distinct thing that has a stable and finite quantity.

This is still what is taught, because it is correct. The conservation of energy holds for all macroscopic objects, and is conserved across frames. If not, perpetual motion devices could be build to exploit that difference.
You will see that many text books give "equivalence" only in outline. It is a way of working up up the argument to larger concepts, that's all.

I need a better way of conceptualizing energy for all this to make sense...

Let's say in this universe, you want to transform as much energy as possible into heat.

Let's say you can only use the kinetic energy available.

How do you go about analyzing how much heat you can generate, if it is meaningless to quantify the kinetic energy of a system?

How much heat is in the Universe? That is not known, as the recent discovery of dark matter has shown. Some say that we only see the tip of the iceberg. Could be.
However, the difference that may exist will not show up in Newtonian mechanics, any more than time variant relativity.
Before the 'big bang' theory, the Universe was thought to be in 'steady state', as expressed by Fred Hoyle. This knowledge, or lack of it, did not seem to bother Brunnel.

 

[spork]

Why the hell is momentum conserved when objects lose kinetic energy due to heat?

Some excellent answers have already been posted, but I'd like to take a crack at this from a slightly different angle...

We can think of any system of particles as an "object" if we wish. At any time T, that "object" has a center of mass. That center of mass is either stationary or moving along in some direction at a steady rate.

We know that in the absence of an external force, any object will either remain stationary, or will continue moving in its same path and direction.

When our two lumps of clay collide, there are no external forces. Remember we're looking at the entire system of particles as a single "object". Therefore, the force of collision is an internal force. As such, the center of mass before and after the collision can't change its course. It will either sit still where it always was, or will continue moving along the same path at the same speed as before the collision.

The momentum of that center of mass is simply the sum of the momentums of the particles that came together. No external force, no change in course for the center of mass of our system.

Why is kinetic energy NOT conserved? Because total energy IS. When those lumps of clay all collide, some stuff is going to go down where this all happens. That's going to create heat. Since heat is energy, and TOTAL energy is conserved, we must have lost some kinetic energy.

Different explanations click for different people. If this helps - great. If it confuses things further - ignore it and go with one of the other explanations given.

 

[humber]

humber, let's make this very, very, very simple.
1) there is an equation for the drag force. According to the wiki, it's F_drag = c(u) u², where c(u)>0 is a coefficient that is nearly constant (but may weakly depend on u) and the force is always in the opposite direction as u.

Yes. it is in the medium. Your interpretation of the equation is wrong.

You say F_drag is the drag of the water upon the object from the water. Because f= ma, and for all non-zero value of F_drag, the object will accelerate to waterspeed, u = 0.
This interpretation is wrong. F_dragis the force required to achieve a velocity u.
You have made that force indefinite.

You apply your fallacy here:

2) u is the velocity of the object relative to the fluid it is immersed in.

3) F_drag=m a assuming no other forces are acting, which means u will be changing with time unless F_drag=0

4) F_drag=0 if and only if u=0, using the equation above

5) therefore, any object placed into a fluid with non-zero u will approach u=0 (like 1/t, if you solve the equation for c a constant), and any object with u=0 will remain so.

This is absurd. It implies
(1) All objects regardless of drag, ( all values of c(u)>0) reach waterspeed.
(2) There is no force F_drag=0 at waterspeed.

So, a half-filled oil drum, will travel as fast as a sleek canoe, and will require no work to keep is at waterspeed. ?

The ideas is false because your mathematics is bad. What you have done is demonstrated that F_drag = c(u) u², or 0 = 0.

Apply this to ohm's law. V = IR.
When V = 0, I =0, so, R =0

Ridiculous.

OK, that's more than enough. You're a liar and a troll, and I've stopped being entertained by this stupid exchange. You refuse to admit your manifold and basic errors, contradict yourself in consecutive sentences, ignore mathematics when it agrees with common sense and demonstrates what a fool you are, respond to questions with gibberish and lies, and maintain your pigheaded ignorance in the face of thousands of posts and tens of posters trying to help you. You're humber than I thought possible, and far too humb to bother with further.

The only cure for trolls is to starve them to death.

Really. I say put-up or shut-up is for those who make extraordinary claims and the don't support them. I have three times asked you for evidence that objects flow downstream at the speed of the water (or other medium), as you claim. I have looked. I find none. I say you are avoiding saying you are wrong because that would undermine the concept of the treadmill.

Don't try your lame and borrowed 'flip-flop' idea. I have made my position clear.

 

[spork]

Why the hell is momentum conserved when objects lose kinetic energy due to heat?

I forgot to mention one very important point...

If you want to gain an understanding of such physical principles, read and think about most any of these explanations. Get your head around the ones that seem to make more sense to you. Even just make your own random guesses if need be. But for GOD'S SAKE don't use humber's posts for anything other than entertainment. I know it may seem like I'm intentionally being silly here, but I'm not. The worst thing someone trying to understand physics could possibly do is to study horribly misconceieved and completely innaccurate descriptions. If humber's explanations were sold over the counter they would absolutely require a warning label "To be used for entertainment purposes only"

 

[humber]

Some excellent answers have already been posted, but I'd like to take a crack at this from a slightly different angle...

We can think of any system of particles as an "object" if we wish. At any time T, that "object" has a center of mass. That center of mass is either stationary or moving along in some direction at a steady rate.

Center of mass is figurative, not an actual property of the body.
"The center of mass of a system of particles is a specific point at which, for many purposes, the system's mass behaves as if it were concentrated."

We know that in the absence of an external force, any object will either remain stationary, or will continue moving in its same path and direction.

Newton, yes. But the force can be applied, yet the object will not move. Like a something on a table. The table provides an equal but opposing force to that of gravity.

When our two lumps of clay collide, there are no external forces. Remember we're looking at the entire system of particles as a single "object". Therefore, the force of collision is an internal force. As such, the center of mass before and after the collision can't change its course. It will either sit still where it always was, or will continue moving along the same path at the same speed as before the collision.

No.
"The center of mass of a body does not always coincide with its intuitive geometric center, and one can exploit this freedom. Engineers try to design a sports car's center of gravity as low as possible to make the car handle better. When high jumpers perform a "Fosbury Flop", they bend their body in such a way that it is possible for the jumper to clear the bar while his or her center of mass does not."
The collision depends very much on the distribution of mass. Objects may become deformed during collisions. An ice skater may draw in her arms to increase her rate of rotation, objects may change too or spin.

The momentum of that center of mass is simply the sum of the momentums of the particles that came together. No external force, no change in course for the center of mass of our system.

Momentum is conserved, yes. Newton, again. Center of mass can change.

Why is kinetic energy NOT conserved? Because total energy IS. When those lumps of clay all collide, some stuff is going to go down where this all happens. That's going to create heat. Since heat is energy, and TOTAL energy is conserved, we must have lost some kinetic energy.

Total energy is conserved, of course. Some KE is lost to heat, sound or light.

Different explanations click for different people. If this helps - great. If it confuses things further - ignore it and go with one of the other explanations given.

Choose option B


A false argument, that adds nothing.
For two items in the same frame, they must have the same velocity. Kinetic energy cannot be exchanged under these conditions. You are describing something that cannot occur when objects are at the s"ame velocity."

The idea is both inert, and limited to a small number of possible cases.
What about a spinning brick and a stable one? If they are traveling at the same nominal velocity, only the center of rotation (or center of mass) of the rotating brick can be said to be in the same frame as the stable brick.

 

[sol invictus]

This interpretation is wrong. F_dragis the force required to achieve a velocity u.

If you said "maintain" rather than "achieve" you'd be correct.

So, humber, what's the force required to maintain u=0?

Ridiculous.

Indeed.

Really. I say put-up or shut-up is for those who make extraordinary claims and the don't support them. I have three times asked you for evidence that objects flow downstream at the speed of the water (or other medium), as you claim. I have looked. I find none.

You've been given about four references so far, and ignored all. You did exactly the same with reference frames.

Enough of your stupidity. Starve.

 

[humber]

]"To be used for entertainment purposes only"[/I][/B]

I agree. You are a clown.

 

[spork]

Enough of your stupidity. Starve.

I wish I had more room in my signature for favorite quotes. Unfortunately the sig is quite limited. Perhaps I'll have to start cycling quotes through.

 

[humber]

If you said "maintain" rather than "achieve" you'd be correct.

So, humber, what's the force required to maintain u=0?

So you think that the object requires force to achieve its position but not maintain it? I bet you think can turn off your car's engine when you reach downwind windspeed.

You've been given about four references so far, and ignored all. You did exactly the same with reference frames.

No, you have no provided any references that support your conclusion that all object achieve waterspeed, regardless of c(u), and require no force.
The only known objects that come close are those that can be dragged by the water surface itself. This is a very limited set of objects, for which Rayleigh's equation does not apply. Of course, I need to remind you that Rayleigh's equation is a simplification that denies your conclusion, and one of many factors in the rather large field of fluid dynamics.

Rayliegh's equation is the externally applied force required to drag an object through a fixed medium. "Gallilaen Relativity" can be then used to say the (u) represents the relative velocity of a fluid, w.r.t an object. Thus, to achieve watespeed, there must be the same level of force as the first case.

Because that external force is the water itself, and the fluid ahead of the object is viscous, work must be done to accelerate that object. This means that the object cannot reach waterspeed.
There are many forces at work, and not simply one. Bluff objects are different from streamlined objects because their dynamic pressure is closer to their static pressure than the latter. And so forth.

The links provided so far are inadequate, because they are simplistic.
Once again, please show direct and unequivocal evidence that objects can do as you say.
Given the wide berth your interpretative gives, it should be easy, being common sense, as you say.

 

[humber]

I wish I had more room in my signature for favorite quotes. Unfortunately the sig is quite limited. Perhaps I'll have to start cycling quotes through.

Like you recycle others' ideas.

 

[Michael C]

http://surendranath.tripod.com/Applets/Kinematics/BoatRiver/BoatRiverApplet.html

 

[humber]

<snip>

(1) The water molecules already have the required velocity. That is what defines waterspeed.
(2) Hot air balloons can use their heat engine to gain altitude. This can be used to gain lateral velocity over and above the wind.
(3) The River boat link is one of countless examples of relative velocity between observers. Not about fluid dynamics, or object speed in media.
The given velocities are nominal. If you deduce from that, no wonder you are in a muddle.
(4) One correct. Waves are indeed not the same.

I would cut down on the waffles.

 

[humber]

http://surendranath.tripod.com/Applets/Kinematics/BoatRiver/BoatRiverApplet.html

Relative velocities, again. Not fluid mechanics. Could be treacle and not water, but still full of your red herrings.
BTW. I said I looked. I have seen all of the links so far posted.

 

[mender]

Very good link, Michael. It shows very clearly what is happening, and also what is meant by everyone else in the world but humber when talking about the velocity of the boat. It is relative to the water.

This means to everyone else that when an object is at "waterspeed", it is just floating along in the water. It is at rest with respect to the water, it has no velocity relative to the water, etc., and it is only the water that the boat's progress or lack thereof is being measured against.

Humber does not use this definition of waterspeed, whether intentionally or otherwise. His definition of waterspeed is a little more convoluted than that. To understand what "waterspeed" is to humber, remember that he references everything to the ground and then uses that as an absolute.

Case in point: a boat on a river. The river is flowing at 5 mph. The boat is floating, i.e., what everyone else thinks of as being at waterspeed. All agreed? (Except humber of course).

Humber however sees that the river is moving at 5 mph. In order for the boat to be at waterspeed (according to him), it has to come to a stop relative to the shore and then accelerate to 5 mph against the current! That condition is what he calls "waterspeed".

His reasoning has been consistent (and wrong) from the very beginning of his posts. He claims that the boat must have 5 mph worth of KE to match the KE of the water flowing by the shore. To admit anything else would destroy the foundation of his argument against the treadmill, therefore there are no links or valid displays of logic that will change his mind until he can understand the proper definition of waterspeed.

His definition is wrong. That is why his answers make no sense. He is either hiding behind this difference in definition intentionally or he really can't understand what it means.

I suspect the latter.

 

[humber]

The link speaks only of relative velocity, Mender. Not relevant.
My definition is like yours, and if you follow the argument with Sol_invictus, like his. All the same.
Nothing to with relative KE, only velocities and forces.

ETA;
The idea that all objects may reach waterspeed, and yet require little or no force to maintain them in that state, is a recasting of the treadmill, but using a different medium.
It also parallels your claims for t he battery-powered car.
I have said that I will post more on the treadmill, but I see that apart from trivial examples of relative motion, no support for the waterspeed claim has been presented.

Why should I make that effort, only to have that ignored too?

So, what is the problem? Not looking, or can't find it?