studiot

Don't worry about static v dynamic friction, the calculations works the same way for both. That is why I just used mu and assumed the user would select the appropriate coefficient. Back to the formula. It says F is a variable that depends upon a constant (mu) and a variable (N) in such a way that F is proportional to N and mu is the constant of proportionality. This is an experimentally verified fact for many situations, including all those in basic courses. Now if we change N by going to the Moon and changing g then we will find that F has changed to maintain this proportionality. If, instead, we remained on Earth and changed N by changing the weight edit : mass ( say for instance we had a box that we gradually filled with sand) and measured F for different values of N we would find the same proportionality. Does this help?

Y Hello Gareth, your difficulty is simply resolved. You will find the push force to overcome friction lower on the Moon. This force may be 313N on Earth, but on the Moon it will be less (wouldn't it be fun to try the experiment?) Remember [math]F = \mu N[/math] The coefficient of friction (within sensible limits) is constant so If N varies, then F must vary. B the way you realise that you should use the normal reaction, not the weight to calculate F? Not doing this is a common mistake, that will lead to errors on inclined planes or pressure connections.

I am starting this as a diversion from the increasingly silly thread about flight, but the discussion is meant to be applicable to both liquids and gases. The heading states the question. Answers have implications for the correction to weighing in air, ships stuck on sandbanks, slip gauges, offshore vertical pipes, dams ... the list is long and varied.

So are you suggesting that 15 years ago balloons rose for one physics reason, but today they rise for an entirely different one?

The aggregated (inter)actions of vast numbers of participants over a wide range of inputs or conditions is the stuff of modern Science and Engineering and the results pretty impressive. But this thread is about 'predicting the unpredictable', which implies inputs or conditions outside the usual or studied range which include one-off Titanic like events or walking out of a clinic and being hit by a bus. Both of these events are conceivable beforehand, but are their occurrence predictable? Anothe situation might be events which are not conceivable beforehand, such as the Comet disasters in the 1950s. Predicting these would have required an evaluation of the question 'What happens if we get our sicence wrong due to factors we do not know about?'

Note the question Talos asked of these authorities is not the basis of his thesis in this thread. So their answer to an innocuous question is not suprising, but really a side show to the main issue.

Unfortunately this takes a deal of work is is often not well received. BTW I'm not arguing against a God slot per se. Just that I was pointing out the number of views and replies to such thread v the number on interesting things such as the self healing concrete that was announced today (not here) . We have quite a few chemists/chemical engineers/earth scientists here but materials science seems the poor relation.

Talos, What relevence do your posts#163 and #166 have to our discussion? Or have you decided to discontinue it, without having the good grace to tell me?

In the UK financial products are required to carry the following 'health warning'. 'Warning any indication of past performance is no guarantee of future performance' Any analysis such as yours can only undertake an analysis of past performance. That is not to say you have have not pushed the level of sophistication forwards, I think you have so well done. But all you can do when using the past to predict the future is assume that the same processes that produced the past results continue to act in the same way and no new unexpected events or processes occur. You can hope to reliably predict the course of a heart condition, but cannot predict that the patient will will out of the clinic and be run over by a bus. I think this is what Dekan was getting at in his comments. New events or processes are more likely with the stock market than medical or geophysical conditions. So yes, progress is being made.

Like Strange, I read the thread when initially posted, but not the linked rules (which I think your zero count refers to?). Also I read the locked thread first, and gained the impression that here this thread was identical except for the locking, but did not see anything I could usefully contribute at that time. This website is entitled Science Forums and I guess and hope that this is the beginning of a cleanup aimed at bringing in more real science overall. If so, I would applaud. Too many times recently I have logged in to find no science posts in the what's new section but several over well patronised threads entitled Jesus Christ this or Jesus Christ that............. Jesus Christ! Is this a scientific or religious website?

Since you can make any nand gate from nor gates this can't be true unless it is also true of nand gates. In fact both nand gates and nor gates are 'universal' gates. http://en.wikipedia.org/wiki/NOR_logic

Studiot. You are forgetting, or deliberately ignoring the fact that there are 3 terms on each side of the inequality—which leads me to ask “what part or parts of the inequality: m(air)gh > m(helium)gh is factually incorrect?” Thank you for that insulting remark (underlined). I am quite capable of performing two simple multiplications and reaching the correct conclusion that when each of the multiplications contains a multiplication by a zero term, then both products are zero and therefore equal and therefore one product cannot be greater than the other. I also explained this in the original. _____________ Studiot, you say: A body continues in its state of rest or motion unless acted on by an external force. I agree. There is no mention of potential energy, or any requirement that any potential energy possessed by this body be used, or that the body must possess any particular potential energy. I agree again, but I do ask, what do you think the force is comprised of. Discussing that force is definitely my offer, but in the same vein as your comments to John Cuthber, I do not propose to extend to that discussion until we have disposed of previous misunderstandings. A body can still suffer change of motion with no change of PE and Newton's First Law still applies. I agree again. Equally a body can suffer change of PE, with no change of motion. I disagree. If a bodies potential energy changes it must either go up or down. There are several mechanisms by which this may occur. A horizontally travelling body will encounter variations of g as it travels. This will not alter its trajectory and therefore h, but will result in a change to the product mgh. This may not be a large effect, but it is not non existant and still obeys Newtons laws. I noted in a previous post that gravitational PE is not the only sort of PE. A balloon in particular will have elastic strain enery, which is another place the initial inflation work is stored. A side vent which causes horizontal motion will release some of that elastic PE to kinetic energy. Of course I can simply give the body a charge and let it aquire electric or magnetic PE and then switch off the field. That again would change its PE, without altering h. There is eventual change of PE in this case, but at the outset of motion, no PE is exchanged. I think you mean—before the onset of motion, in which case I agree again. I said outset, because I meant outset. This is before motion occurs. But hey, what's the difference of a couple of letters between friends? Although the point appears trivial, it is in fact important because the force must exist before any energy is transferred. In fact that force can exist indefinitely in a tethered balloon, with no energy exchange taking place at all. That is PE only changes as a result of work being done by the external force. I agree yet again—but I ask again, what is the external force comprised of? The external force is the resultant of all forces acting on the body, considered as a free body that is not in equilibrium. Please indicate whether you understand free body diagrams in mechanics so we can discuss this further. I will explain them if necessary.

Discussion at last. +1 Newton's First law : A body continues in its state of rest or motion unless acted on by an external force. The balloon rises, ergo it is acted on by an external force. Notes 1) There is no mention of potential energy, or any requirement that any potential energy possessed by this body be used, or that the body must possess any particular potential energy. 2) A body can still suffer change of motion with no change of PE and Newton's First Law still applies. 3) Equally a body can suffer change of PE, with no change of motion. 4) There is eventual change of PE in this case, but at the outset of motion, no PE is exchanged. That is PE only changes as a result of work being done by the external force. Perhaps together we can answer John Cuthbers' question about the generator. You just need to get the full and complete sequence of events in order for the flight of your balloon.

Actually it does. Here is one of the factually incorrect statements you made It is factually incorrect because at ground zero h = 0 so the inequality you state is factually incorrect. Since h = 0 and PE = mgh, the potential energy for air must equal the potential energy for helium since they both must equal zero. I have done you the courtesy of accepting the potential energy scale and zero point that you have consistently used throughout, and agreeing with you that on this scale PE increases with height above ground level. This was explained in my post #139 and more fully in the reference I gave there. A further question for you, If I lifted the carbon dioxide filled balloon by means of a crane, Air would flow in to fill the void. At least some of this air would come from above, just as with the helium filled balloon. You say (and I agree) that this descending air would thereby loose some of its gravitation energy. So where would this PE go to, since it is no longer required to 'lift the balloon'?

Good evening, John. Once again Talos is avoiding the question, as predicted - ain't scientific theory predictions wonderful? That's a very posh explanation of weight. How about simply pointing out that Archimedes (whom Talos has already invoked several times) was the first to distinguish between real and apparent weight?

Yes we are all agreed on that. He also tried to avoid the question every time someone asked a searching question about his statements. I particularly noted his attempts to avoid answering swansont's question about the closed room.

Odd, but I could have sworn that you introduced the balloon powered generator, although Talos introduced the balloon.

Perhaps, but I would say the length of the string was more significant than the size of the balloon.

I agree Science is about discovery, but surely you cannot discover something that is incorrect? I would add that Science is about a whole lot more than mere discovery, important though that is. Words or phrases like application, codification, connection to other discoveries, recording, postulation, prediction come to mind.

Although there is energy stored in the balloon, this is not the energy to fly. This energy is not stored in the balloon. Will a balloon rise in a vacuum? Agreed, which is why you should specify your baseline. This applies to any object with mass, including helium balloons, not just rocks. Yes indeed and the work against air friction has the same source of energy as the work of electricity generation during the ascent. This source of energy springs from an agent external to the balloon. No it's quite clear by both the equations you quoted and since the source of energy is not the gravitational PE, by whatever measure, the gravitational energy is unaffected by work done.

I also said, a while back, that Talos and John are talking about different things in the GPD (Great Potential Energy Debate) Both views are presented here. http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html#gpt and again here http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/gravpe.html#c1 So, because of a difference of origin both Talos and John are correct and gravitational Potential Energy can be regarded as positive or negative to suit. However both must agree that it takes external work to lift an object, even a helium balloon, from the surface to height, h, whether you count that work positive or negative. The important fact is that (from the equations) that work is the same whether the balloon is contrived to generate electricity or not in the course of its ascent to h Further h is quite independent of the quantity of electricity generated.

And John has consistently maintained this throughout the thread. The difference is he has offered a reason for his view. You simply reiterate yours without support as yet again in the quote above. Moreover you have never once offered a view as to where the energy of the generator comes from, if not from the gravitational potential energy of the balloon, although you promised to do so and have been repeatedly asked for this. Way back, I even agreed with you, but I have been waiting for you to show that you understand the mechanics of your statement and are not just blindly quoting others. I am still waiting.

But some observations are more exact than others. Take the observation "the colour of X is.........." Now orange is, by definition, the colour of that particular fruit. That, to me, is an example of the strongest available truth. But what is the colour of my bedroom wall? Well the paint manufacturer called it April Yellow, but another manufacturer calls it Spring Sunshine and yet another just used the British Standard colour number, called cream. In theory I will get the same ccolour, whichever product I choose.

Canadians +1

If there's a leak in my bath plug does it follow that I can't fill my bath?