# Kedas

Senior Members

695

## Posts posted by Kedas

### Help! Moment of inertia??

Not sure why you talk about 2D formula's, there are none for MOI.

You know very well that it is not possible to have a mass without a volume.

Maybe on some quantum level but I'm sure these formulas don't apply there.

Even if the height is only dh it would still be a cylinder.

And simplifying the formula for certain applications is ok for me but that still doesn't mean that a circle has a MOI.

Unless you like to change the definition of a circle that it has a height.

What you are saying is as interesting as saying that a circle (r>0,L=0) does or doesn't have an electrical resistance. if there is no distance to travel there no meaning in the word electrical resistance.

### Help! Moment of inertia??

Kedas, while physically being merely a 2-D object a circle or square wouldn't have any mass in the real world, it doesn't prevent us from mathematically talking about a circle or square's MOI. If you want to start down that road, why not just say that a circle or square can't have an MOI because there is no such thing as a perfect circle or perfect square?

Mathematically we can assign an area density or mass per unit area and make it mathematically have mass. Just like mathematically we can and do use point masses and point charges all the time, despite there being no such thing in the real world. It is a simplifying assumption made primarily to make the math easier and get a result that is going to be very close to the real world. To a very large extent, it doesn't really matter at all that no 2-D object has mass, or that there is no such thing as a perfect circle, mathematically such objects do exists and we can perform mathematics on them.

From what I read genralz isn't interested in math for the math.

The simple fact is that the MOI formula requires a mass to have some meaning.

Yes you can make up an mass that has no meaning, I don't care.

genralz, in case you mean MOI of a cylinder or cubiod etc.

here are the formula's.

http://en.wikipedia.org/wiki/List_of_moments_of_inertia

### Help! Moment of inertia??

A square or circle doesn't have a volume/mass therefore it has no moment of inertia.

used if you want to know how much energy is in there.

You mean

http://en.wikipedia.org/wiki/List_of_area_moments_of_inertia

"The second moment of area, also known as the area moment of inertia or second moment of inertia "

used if you want to know how easy it will bend and where.

### Physicists resolve a paradox of quantum theory

I didn't find the news forum to post it in.

Anyway, I was wondering if this is related to the Heisenberg's Uncertainty Principle or am I understanding it wrongly?

### Physicists resolve a paradox of quantum theory

http://www.physorg.com/news151164690.html

"For nearly a century, the widespread interpretation of quantum mechanics suggests that everything is uncertain until it is observed, and that observation inevitably alters reality," says Professor Steinberg. "However, in the 1990s, a technique known as 'interaction-free measurement' seemed to promise the ability to 'see without looking,' as a Scientific American article put it at the time. But when Lucien Hardy proposed that one could never reliably make inferences about past events which hadn't been directly observed, a paradox emerged which suggested that whenever one attempted to reason about the past in this way they would be led into error."

### Spooky wave interference

if electromagnetic radiation started flowing in reverse, back toward its source, it could cause serious confusion. But this is just what Cesar Monzon, a Senior Scientist at Enig Associates, Inc., in Silver Spring, Maryland, has discovered. In a recent study, he theoretically demonstrates that, under special conditions of geometry, location, and frequency, power may flow backwards in the direction of its source. The study is published in a recent issue of Physical Review Letters.

http://www.physorg.com/news141394051.html

### Chance to see a major asteroid impact. On Mars.

The Martians will zap it first with their ray guns.

no impact:

http://neo.jpl.nasa.gov/news/news156.html

They must have used their gravity beam ;-)

### Chance to see a major asteroid impact. On Mars.

If they don't know yet if it will impact mars then a close fly by could change the orbit of it so it is on track to make a collision with an other object, maybe earth in 2011.

### Chance calculation

Thanks uncool.

My weird 'feeling' example feels much more right now

### Chance calculation

Thanks for the explanation

1-((n-1)!/n!) looks more right because it doesn't look too simple

### Chance calculation

It looks right for n=2 and n=3

but n=1 should be 1.

and if n is very large than the change that at least one stays at its current position is very high.

So if I have a cube of water and give it a good (random) shake.

Then I can say it is very likely that at least one molecule arrived on it's previous position.

That sounds a bit weird?

### Chance calculation

If you have an ordered list of numbers and you randomly change them what is the chance that at least one number will stay on its previous position?

Or more practically if you give everyone a number and you let them blindly pick a number. What is the chance that at least one persons picks his/her own number again.

### Evolution more predefined than we think

Evolution is deterministic, not random, biologists conclude from multi-species study

A multi-national team of biologists has concluded that developmental evolution is deterministic and orderly, rather than random, based on a study of different species of roundworms. The findings are reported in the latest issue of the journal Current Biology.

http://www.physorg.com/news114700358.html

So random changes with natural selection is not reality, but more like a smart change.

### Water forms floating 'bridge' when exposed to high voltage

From the page:

The bridge forms due to electrostatic charges on the surface of the water. The electric field then concentrates inside the water, arranging the water molecules to form a highly ordered microstructure. This microstructure remains stable, keeping the bridge intact.

How high the static voltage has to be or how pure the water has to be is not mentioned, but they do say that dust is probably a reason that makes it unstable

It's like liquid ice.

### Water forms floating 'bridge' when exposed to high voltage

http://www.physorg.com/news110191847.html

It's cool but weren't we suppose to find something like this about a half-century ago??

I'm mean you don't even need a lab to reproduce this experiment.

### Laser TV/projection technology

actually, laser projection will increase the amount of heat rediated which may decrease product life and then more heat sinks!!!

Uhm, you didn't read any of it didn't you?

Energy consumption will decrease and MTBF increases strongly.

20.000 hours (compared to 2000 lamp hours with current projectors TV's)

That is almost 7years for 8 hours every day, by then I will buy something else.

P.S. you don't need heat to make laser light.

### Quantum computing more practical now?

http://dwave.wordpress.com/2007/01/19/quantum-computing-demo-announcement/

The Orion system is a hardware accelerator designed to solve a particular NP-complete problem called the two dimensional Ising model in a magnetic field. It is built around a 16-qubit superconducting adiabatic quantum computer processor. The system is designed to be used in concert with a conventional front end for any application that requires the solution of an NP-complete problem.

I'm curious...

Some info:

http://en.wikipedia.org/wiki/NP-complete

http://arxiv.org/PS_cache/quant-ph/pdf/0206/0206003.pdf

### Laser TV/projection technology

Hi,

It seems that new LED technologies will be past by laser technology in the display market.

They even claim the end of plasma TV's. (due to better quality with lower production cost)

lasers: http://www.novalux.com/

Making a big investment in a new plasma screen could be a bad idea now.

They claim to have a first commercial model end 2007.

### Who believes in evolution?

I was just thinking how many people (%) Are actualy educated enough to interprete the presented data and label it true or false.

30% is probably already too much.

So that means that 70% of the people base their decision on other people's decision....

### Who believes in evolution?

More interresting than how many say it's true is are their numbers increasing or decreasing.

The US is decreasing. And there are still much people who can't decide.

Not sure if that is because they didn't understand the question or if they are in doubt.

### good laptop for student

do you guys think that the 2GB ram upgrade on the macbook pro is worth anything more than bragging rights?

If you later want to install vista on it then it will be a must, now it's just nice for some not common situations.

Anyway AMD & Intel started their dual-core price war.

Intel made a faster CPU than AMD (since long), AMD is dropping prices fast, Intel also but mostly to get rid of their old CPU.

So if you are not in a hurry...

I'd recommend a Riva TNT2. Actually' date=' I'd recommend starting again from scratch as you could get something at least three times as powerful for under £300.

You don't actually have to use a DVI-monitor with DVI-out. There are adapters.[/quote']

a TNT2 that is one generation too far in the past, that is probably what is already in his/her PC.

And yes almost al cards still have an analog output for your monitor.

Note: Intel will soon come out with a good dual core processor so they started dumping their old P4 ones (although I don't advice buying such an oven)

planned release in July.

You will probaly have an old AGP 4X graphics card slot.

I'm even not sure that there is an upgrade for you.

Why would you want to upgrade your graphics card, You do realise that a fast card will never reach its potential because your CPU an memory can't follow.

(I case you didn't understood it, I'm saying: buy a new PC)

Maybe you can get a Nvidia Geforce4 Ti (not MX) on the second hand market This is a DX8 card.

I don't think that any DX9 card will work (well) in your PC.

http://cgi.benl.ebay.be/Abit-Siluro-NVIDIA-GeForce-4-Ti4200-AGP-128MB-graphics_W0QQitemZ8817886930QQcategoryZ27387QQrdZ1QQcmdZViewItem

(You don't really lose a lot if it doens't work wel)

P.S. You probably mean 1Ghz cpu clock or 1GB memory instead of 1MB

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