Dan O.
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Anyone that has read Alice knows that you have to run as fast as you can just to stay in the same place.
Split Thread The validity of classical physics (split from: DWFTTW)
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Anyone that has read Alice knows that you have to run as fast as you can just to stay in the same place.
That's as close to anyone saying they are wrong, as I am likely to get.
I meant the left hand side of the equation. The force.
Yes. My question was rhetorical, saying, if you think that force is only there while accelerating (zero at windspeed) then why don't you think that it is not proportional to mass?
Sol_invictus seems to think that. The right hand side ( of the equation) is not the applied force but the resultant reaction to it.
Yes, but if your refer to the previous misuse of Rayleigh's equation, the force is not specified in the way that you just did, but simply assumed to be there so as to satisfy the equation. That was my point.
Remember, that was Sol's rebuttal to my assertion that he had made that assumption, and indeed he did.
That is the force of the wind. It drives the balloon forward in the direction of the wind. That is what that equation means . There is no need to discuss the mean free path of the molecules that strike the balloon to infer the net force.
Yes in this case;
... its state of rest (tethered)
....uniform motion (final velocity)
The idea that there is no relative motion at winspeed is actually not "Equivalence", but Galilean Relativity. Since that time, we know that such an ideal state does not exist, but is approximated. There is always turbulence around an object in motion. Only recently has CFD allowed that to be modeled. At the very least, it is that residual drag that stops the balloon from reaching windspeed.
You are suggesting motion without force.
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Alert to all: Hypothetical statement ahead.
It could be argued that an object driven to terminal velocity, say a car against a headwind, is always accelerating. It must be. If is doesn't it will slow. If it slows just a little for just a short time, then there must be a force to accelerate it. The integral is the applied force that is required to maintain that velocity.
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This is how wind works. There are constant collisions between each and every air molecule, but there is a net drift force in one direction. There is a constant force driving the wind ( the energy being provided the sun), and that is the same for wind borne objects.
Rayleigh assumed that all momentum that impinges upon the objects effective area is transferred. ( He assumes that exhausted fluid come to a stop). That is critical. He recognizes that in effect, the 'spent' fluid is replaced by new 'energetic' fluid. Motion in fluid is a constant exchange of energy that is replenished by what is available. It is a sea of momentum, not a simple force.
Words confound the issue. If you understand how the formula in the balloon article is derived, then you would see the point. If you made an electrical analogue, the point would be clear to you in seconds. It is an extremely useful and lucid notation. Do it!
Then get back to me if you disagree with my conclusion.
In the case of the objects going downstream in a fluid, there are two 'types' of 'drag'. One couples the object to the air accelerating it downstream, and the other opposes that motion, but of course, they appear as one net force.
Drag is always dissipative, it is a loss.
I was arguing the idealized case for no drag. Not for the wind force itself.
If you understand the why there is still a force on the object at windspeed, then you will understand the remark.
No that is wrong. If the wind needs force to move, why not the objects traveling with it? Wind is a sea of momentum, it has an effective constant force, the result of a potential gradient. A pressure gradient.
Without momentum exchange, no work can be done on the balloon. (Work = force x distance). What you are suggesting is impossible.
Why does it not continue to accelerate?
The body can perhaps be said to do that, but by the infinitesimal amount of each momentum exchange with each and every random air molecule. Objects in motion in fluids are constantly doing this, but the average force, the drift force, is in the direction of the fluid. This is also true of electrons in wires and water in rivers.
Yes. And a skydiver in the gravitational potential field too? Or do they experience no net force at maximum velocity?
Nope. How can the balloon slow even the tiniest amount, without being once again accelerated? How does it maintain its constant velocity?
According to the Wisdom of Sol_omon, objects travel at windspeed regardless
of drag, yet no work is required. This is contradicted by the equation provided. Only your false interpretation allows the left hand side of the equation to be zero.
All you need to do is understand how that equation, or its basic first principles are derived, and then all will be well. Peharps you think that objects that are in a potential gradient feel no force?
The article supports my claim, and contradicts yours. Too hasty.
Every once in a while you have to read some humber just to recalibrate your sense of the absurd. When you do, you're sure to find gems like this:
I've got to figure out how to add more favorite quotes to my signature!
I've got to figure out how to add more favorite quotes to my signature!
Forest Hump.
Drag is dissipative, a loss. Even if you think that is is "pulling" the balloon or "pushing" against it.
When drag (friction) transmits a force velocity is lost.
Drag = Resistance
Force = Current
Velocity = Voltage
Ohm's law
V = IR
Velocity (difference) = Drag x Force
It's no coincidence. Electrons in a conductor behave a lot like the wind.
You need to change to -80dm quote. DOH!
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Modified
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If they were experiencing a net force, it would not be maximum velocity.
http://en.wikipedia.org/wiki/Terminal_velocity
No one is arguing against the idea that when you put, say, a canoe into a moving river, that the resulting drag doesn't slow the river by some infinitesimal amount, as the mass of the canoe is accelerated to the (now lower by some immeasurably tiny amount) speed of the moving water. Think inelastic collision.
So what? Once the canoe has reached (or become asymptotically close to, if you want to quibble) the speed of the moving water, its acceleration drops to zero, no more force is transmitted, no relative velocity is lost or gained, and the canoe and the surrounding water move in unison.
You seem to have moved into discussions of dancing angels (I will refrain, with no little effort, from any "pinhead" comments.) We claim that (all) floating objects placed in a flowing body of water reach (or approach arbitrarily close to) the speed of that flowing water. You reject this claim, despite it being universally and trivially observed, and the fact that this phenomenon provides the basis for successful air and water navigation over the history of those respective modes of transportation. I'm curious to know the order of magnitude of our supposed "error". Are we talking about parts per billion? Far less? Or do you feel we're incorrect in some non-trivial way? Can you roughly quantify how far apart we (that is, you as compared to, well, everyone else) are in our assertions?
Tunny
John Freestone
Graduate Poster
Yes. Why else do you think it's called "terminal velocity", because they die at that speed?
I refer you once again to the established physics of Sir Issac Newton. No net force ALWAYS equals no acceleration, which always equals constant velocity, whether zero or non-zero.
CORed
Penultimate Amazing
Newton's first law is apparently invalid in the Humberverse.
CORed
Penultimate Amazing
I think you are grossly underestimating the depth of Humbers misunderstanding of basic physics. Fairly early in the DDWFTW thread, he was insisting that the propeller on the cart on the treadmill was generating not thrust but drag. I think in Humberian physics, drag is any force that opposes motion with respect to the one absolute frame of reference, "the ground" and given the current discussion, I think he must believe that drag is present any time there is motion with respect to "the ground".
Then again, it's tough to tell what Humber really believes, as it changes any time he gets backed into a corner he can't extricate himself from.
ETA: I should say any time he realizes he's been backed into a corner he can't extricate himself from, as he his just about always backed into such a corner. Every once in awhile, as with the claim that wheel slippage would improve the performance of the cart on the treadmill, he actually somehow manages to realize he is wrong, and then threatens to put anyone who mentions it on "fastscroll".
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I wish I could take the credit, but actually I was only quoting a certain Mr. Newton.
In your post you used quite understandable english (not typical of you) and made it clear, that you don't accept newtons first law to be valid.
Drag = Resistance, hmmm.... I do see the analogue. The hot air balloons are very colourful. And there are coloured stripes also in resistors. The colour stripes tell us how many Ohms of resistance that component provides. YES! The colours of hot air balloons must tell us how much drag there is!
I still disagree with you. And it takes lots of effort to put it that mildly.
You guys. You mix ideas together if they look similar.
If a body falls from a height, but in a vacuum, it will accelerate due to gravity. Just before it hits the ground the potential energy it obtained from being raised in the gravitational field will now be that body's KE.
Introduce drag, and the body will be accelerated, but not reach the same velocity as in the vacuum, so just before it hits the ground, the KE will be less. The difference between the potential energy and the final KE, being lost to heat by friction. At terminal velocity that velocity is constant, but of course the gravitational force is at a maximum, because gravity is a constant force near the Earth's surface. (It is also true from the third law that the earth is accelerated towards the skydiver, but the differences in masses is huge, that it is negligible. Conservation of energy still applies, so the earth is accelerated the the other way when the object hits the ground.)
That is a case of constant applied force, with velocity dependent drag.
There seems to be some confusion about this.
If you push against a wall, an equal but opposite force is generated. Let's say that is 10N but the object does not move. This is static force.
Apply 10N force to a free body lying on the ground. It will accelerate until the forces of friction against it equal that applied force. The velocity at that point will be the maximum velocity obtainable under those conditions.
Both cases now have equal but opposite forces of 10N, but one is moving. The difference is that one has been accelerated while the forces were not in balance. (Also giving the body KE). To maintain that velocity against the retarding friction, that force must be maintained. That is a dynamic force.
Once humber catches on to Newtonian physics, it won't be any fun anymore!
And in both cases, the bodies are staying in motion or at rest because the forces are balanced. If you remove both forces (no fair trying to remove only one!), the object doesn't accelerate.
Maybe it would be easier to think about what would happen out in space. As long as one force is applied to an object, the object will accelerate.
And in both cases, the bodies are staying in motion or at rest because the forces are balanced. If you remove both forces (no fair trying to remove only one!), the object doesn't accelerate.
Maybe it would be easier to think about what would happen out in space. As long as one force is applied to an object, the object will accelerate.
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sol invictus
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Well, now we know that Forrest Hump doesn't know what "acceleration" means, either.
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spacediver
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CORed
Penultimate Amazing
In that case, I think it's going to be fun for a long time.
The river is so massive, that the slowing is considered to be negligible.
Not a quibble. Yes, it has stopped accelerating, but that does not mean that there is no force. Make the boat out of water. It occupies the same mass and volume. Were it it be stopped somehow, say held, then there would be the full force of the water behind it. When let go, the water canoe accelerates to the same speed as the bulk of the water, because being just like water it has no drag. Why does it keep on moving? It is driven by the force of the water, like the rest of the water. Part of a moving mass. The force is not zero. That canoe is on the bank, or in some still water somewhere.
I would. (On that point, you all force that situation. It should have been done and dusted when it was clear that the cart balances, and that the belt and ground observers do not agree.)
No way. That is not so. It is far from trivially observed. If objects could be blown to wind or waterspeed with no force at terminal velocity, then why the fuss over the cart? Why cannot a simple sail not reach windspeed?
Quite wrong, really. Firstly the case of 100% windspeed balloon is not possible. I asked for examples of any bodies that do so. This meteorological balloon is not one, and you will not find one. That's one thing.
The other is that you can accept travel without force or work. Now show me objects that do that. Travel without work. Not on this planet. This is the major error.
Please take a look at the meteorological balloon formula.
What you say, is that the object will be accelerated by the force F until V-Vb = 0, when the force becomes zero. If the balloon is stationary, then V-Vb is maximum, so F is maximum.
That is not the correct interpretation. What is said, is "that a given force can accelerate a specific object to a certain value of V-Vb."
If there is no drag at all, then the force will accelerate the object to V, and that force will hold it there. If not, then Vb will be lower than V, but F (may) still the same, because perhaps Cd, or one of the other variables is bigger.
(Remember, that if an object is dragged through air by force, the drag increases, by the square of the velocity, because then relative and absolute velocities are the same.)
Say you have a sleek car, and blocky car driven in air. For the same horsepower engine, which will reach the higher terminal speed? The sleek car, of course, but you can't turn the engine off in either case. One is just going faster for the same power.
In a fluid, it means that if the object is sleek, there will be enough force available from the river, to drive it closer to waterspeed, but in each case, like the two cars, the force is still required to maintain that velocity.
That force is the force available to the object by virtue of its shape and many other factors.
The balloon equation is for both the driving and retarding forces. In this case, the values of mass and Cd and A and p, allow a solution that is perhaps close to V.
Despite the label hypothetical argument and the underlining of could ?
It's called "working up a argument" .
This is different from getting worked up because your argument is wrong.
"The Third Law means that all forces are interactions, and thus that there is no such thing as a unidirectional force. If body A exerts a force on body B, simultaneously, body B exerts a force of the same magnitude body A, both forces acting along the same line."
The skydiver does move the Earth, but only very tiny bit. Don't worry though, you won't feel a thing.
You may need this:-
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l3b.html
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