# The Naked Scientists Forum

### Author Topic: Question of the Week - Old Version  (Read 176912 times)

#### ukmicky

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##### Re: Question of the Week - Old Version
« Reply #325 on: 12/02/2006 16:53:54 »
Doc your back, nice to see

Michael

#### sharkeyandgeorge

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##### Re: Question of the Week - Old Version
« Reply #326 on: 13/02/2006 20:04:08 »
i have broken many many tarzan swings and 90 percent of the time the rope breaks not at the top of the ark but just as you start to swing back some thing do do with the change in centrifugal force perhaps?

"Defender of the Sea"

#### chris

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##### Re: Question of the Week - Old Version
« Reply #327 on: 27/02/2006 17:15:48 »
Ok, this was a hard one to explain, but here's the solution to:

"AT WHAT POINT IN ITS ARC IS A ROPE SWING MOST LIKELY TO BREAK?"

The simple answer is at bottom dead centre. At this point the kinetic energy of the swing and passenger is at its greatest, and the effect of gravity is acting straight down through the rider on all of his weight. So the swing is most likely to break at the mid point of its travel.

Now for the more complicated proof of concept.

If we consider a swing (and rider) which together have a mass of m suspended on a cord of length L (with negligible mass), swinging through an angle of x degrees (where 0 degrees is the horizontal).

Lets assume that the swing begins held out at 90 degrees i.e. horizontal (admittedly a scary ride!). Starting from this point, the speed of the swing at any point in its subsequent arc of travel will be such that the kinetic energy (0.5 * m * velocity(v) ^ 2) is equal to the change in potential energy from its starting point. The change in potential energy will be:

the mass of the swing and rider(m) * gravity * L * sin(x)

So 0.5 * m * v^2 = m * g * L * sin(x)

hence v^2= m * g * L * sin(x)/0.5 * m

hence v^2= 2 * g * L * sin(x)

Now the pull exerted by the swing on the rope is given by the formula speed squared / radius of arc, or v^2 / L (length of swing rope).

So the pull here is v^2 / L which equals 2 * g * sin(x).

The contribution due to gravity at any point along the travel will be g * sin(x).

Since F (tension in the string)= m * a

then the tension (T) must be m * ((2 * g * sin(x) + g * sin(x))

which equals 3mg sin(x).

So, at any point the tension in the rope supporting the swing will be 3 * mass of swing and rider * sin(angle travelled).

If we substitute into this formula, sin(0) - horizontal - is 0, so at thte start point, when the speed is zero, there is no tension on the rope.

One third of the way to the bottom dead centre (30 degrees), sin(30) = 0.5 so the tension is 3mg * 0.5

Two thirds of the way to the bottom (60 degrees), sin (60) = 0.666667 so the tension is 3mg * 0.666667

And at the bottom (90 degrees), sin (90) = 1 so the tension is 3mg. This is the maximum. So the rope has to be capable of holding three times the rider's weight.

I hope that helps to lay that one to rest.

Chris
« Last Edit: 27/02/2006 17:17:48 by chris »

#### chris

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##### Re: Question of the Week - Old Version
« Reply #328 on: 27/02/2006 17:18:38 »
Okay, here's this week's QOTW:

"WHAT IS A HOLOGRAM, AND HOW ARE THEY MADE?"

Have a go, below.

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#### neilep

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##### Re: Question of the Week - Old Version
« Reply #329 on: 27/02/2006 17:54:04 »
It's a telegram with no substance whatsoever

ps: Well done Michael..I reckon you got the ropey question right...with no strings attached of course !!

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#### DoctorBeaver

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##### Re: Question of the Week - Old Version
« Reply #330 on: 28/02/2006 20:24:38 »
Neil - that was almost bad enough to be mistaken for 1 of mine!

On a serious note, a friend of mine set himself up as a Hollergram. Instead of singing the message to people, he shouted it at them!

#### neilep

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##### Re: Question of the Week - Old Version
« Reply #331 on: 01/03/2006 11:24:49 »
Eth, Mine don't even come close...

Hollergram .....agghhhhh !!..see ?

Men are the same as women.... just inside out !!

#### DoctorBeaver

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##### Re: Question of the Week - Old Version
« Reply #332 on: 01/03/2006 14:11:42 »
Or what about a Holagram who just says "hello" to you in Spanish?

#### Ray hinton

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##### Re: Question of the Week - Old Version
« Reply #333 on: 01/03/2006 15:58:57 »
or a HALOGRAM,just does it while you tend your flocks by night [:0]

its the drugs,y-know.

#### DoctorBeaver

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##### Re: Question of the Week - Old Version
« Reply #334 on: 01/03/2006 16:36:07 »
That's silly

#### ukmicky

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##### Re: Question of the Week - Old Version
« Reply #335 on: 02/03/2006 00:39:53 »
quote:
originally by ukmicky
At the bottom of the arc i would have thought

quote:

Chris

HMMM its not often i get things right but occasionally i astound myself and others and produce the goods.

Where's my congratulations, where's my prize. i want my goody bag

Michael
« Last Edit: 02/03/2006 00:40:53 by ukmicky »

#### clouded.perception

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##### Re: Question of the Week - Old Version
« Reply #336 on: 08/03/2006 06:59:13 »
Charmeleons have different coloured cells in their skin. To change colour, they 'open up' the cells with the correct pigment, thus showing that colour (or combination of colours) on their skin.

I can picture in my mind a world without hate, a world without war.
And I can picture us attacking that world, because they'd never expect it.

#### chris

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##### Re: Question of the Week - Old Version
« Reply #337 on: 08/03/2006 08:52:28 »
That's very nice, but this week's question is about HOLOGRAMS!

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#### JimBob

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##### Re: Question of the Week - Old Version
« Reply #338 on: 08/03/2006 18:09:40 »
Split a laser beam, record the interference pattern made when this beam shines on an object from two different directions. (It's doned with smoke and mirrors.) Project the record to make the same interferance pattern and shine coherent light on it. Presto, a 3D Hologram.

Perhaps this was ignored because it is so easy. These thngs have been around since the 50's. (I wasn't around then - I was a Science Attache at the Klingon Embassy in Star Cluster 4289-F.)

Jim

#### ampwelder

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##### Re: Question of the Week - Old Version
« Reply #339 on: 08/03/2006 19:13:27 »
Hologram come from the greek words "whole" (holo) and writing (gram).
A lazer is used to photograph an object (instead of incoharent-normal- light) it does this in such a way that enough information can be recorded to give the imprestion of a 3-D object. This is an illusion as the holograph is actually only 2-D like a regular picture.

#### NakedScientist

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##### Re: Question of the Week - Old Version
« Reply #340 on: 24/03/2006 08:15:51 »

"WHAT IS A HOLOGRAM, AND HOW ARE THEY MADE?"

Holograms are made using lasers. You need a laser because the light that is produced is "coherent". That is, all of the light waves in the beam are synchronised, and of the same amplitude (size), so they diffract (bend) identically. You can demonstrate this by shining a laser at a prism. Compared with white light, which splits up into its composite wavelengths because each bends by a different amount upon entering the prism, when a laser is fired at a prism just a single band of light is produced.

To make a hologram a laser beam is fired at a beam splitter. This sends part of the light beam, termed the reference beam, to a piece of photographic film. The other part of the light beam, let's call it the image beam, is directed at, and subsequently reflected off, the object which you intend to turn into a hologram. It, too, then shines onto the same piece of photographic film as the reference beam mentioned above.

When the reference beam and the image beam meet at the photographic plate they have travelled different distances and their light waves no longer line-up with each other precisely. As a result they "interfere" with each other with some waves adding together constructively to make brighter patches and other parts of the wave behaving destructively and cancelling each other out to make darker areas. This is called an intereference pattern, and it's what is subsequently recorded onto the photographic film. This is the hologram, the 3D representation of the original object.

TNS

#### NakedScientist

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##### Re: Question of the Week - Old Version
« Reply #341 on: 24/03/2006 08:17:17 »
Ok, here's this week's QOTW:

"HOW DO STINGING NETTLES WORK ?"

TNS
« Last Edit: 24/03/2006 08:39:32 by NakedScientist »

#### wim

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##### Re: Question of the Week - Old Version
« Reply #342 on: 12/04/2006 19:31:30 »
Stinging Nettles ,each of it’s leaves are about 10 cm long, roughly heart-shaped and have large teeth around the leaf edge. They also have tiny hollow hairs on the main stem, leaf stems and on veins on both upper and lower sides of the leaves.
When a human brushes by the plant and it touches their skin, the tiny hollow hairs break off and release formic acid, histamine, acetylcholine, serotonin (5-hydroxytryptamine), plus some unknown compounds. These irritate the skin and cause white itchy spots to appear.

grtz

#### daveshorts

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##### Re: Question of the Week - Old Version
« Reply #343 on: 12/04/2006 20:06:13 »
A hologram is a pattern that when light is reflected (or transmitted depending on the hologram) from it, the light reflecting from the parts of the pattern interfere with each other to produce light leaving as if it came from a 3D object.

Doing this by drawing the pattern from the start is immensely difficult ( although a company in Cambridge is developing a projector that works on a similar idea http://www.lightblueoptics.com/ ) however if you shine light reflected from an object and light coming directly from a laser the two light beams will interfere producing a pattern. Luckly this is the pattern you need to make a hologram.

If you used non-coherent light (light where you can't predict whether it is going to be a peak or a trough from one moment to the next) it would produce an interference pattern that was not stable for long enough to take the photo, so you would actually get an averaging of lots of different interference patterns which won't produce a hologram.

#### chris

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##### Re: Question of the Week - Old Version
« Reply #344 on: 28/04/2006 11:57:27 »
Here's this week's QOTW:

"HOW DOES A SOLAR CELL TURN SUNLIGHT INTO ELECTRICITY?"

Have a go, below...

Chris

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- Groucho Marx

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##### Re: Question of the Week - Old Version
« Reply #345 on: 28/04/2006 12:06:05 »

The Structure of a Solar Cell
Over 95% of all the solar cells produced worldwide are composed of the semiconductor material, silicon. As the second most common element in the earth’s crust, silicon has the advantage of being available in large quantities. Furthermore while the material is been processed, it does not have an effect on the environment. [http://www.solarserver.de/wissen/photovoltaik-e.html] Another reason for the use of silicon for solar cells is that the energy needed to ionize silicon electrons matches well with the energy of photons coming from the sun. If the photons had less energy (if the solar spectrum were more red), there would not be enough energy to free the electrons, and if the photons had more energy (if the solar spectrum were more blue or ultraviolet), then all the energy above what is needed to break the electrons free would be lost as heat. [http://www.astropower.com/how_solar_cells_work.htm]

To produce a solar cell, the semiconductor (silicon) is “doped” or contaminated. “Doping” is the intended introduction of chemical elements, which can obtain excess positive charge carriers (p-conducting semiconductor layer) or negative charge carriers (n-conducting semiconductor layer) from the semiconductor material. If two differently contaminated semiconductor layers are combined, a p-n-junction results.

At this junction, an interior electric field is built up, which leads to the separation of the charge carriers that are released by light. Through metal contact, an electric charge can be tapped. If the outer circuit is closed, meaning a user is connected, then direct current flows. A transparent anti-reflection film protects the cell and decreases the reflective loss on the cell’s surface. [http://www.solarserver.de/wissen/photovoltaik-e.html]

What you do speaks so loudly that I cannot hear what you say.

#### razorbill

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##### Re: Question of the Week - Old Version
« Reply #346 on: 14/05/2006 23:19:25 »
I have'nt progressed that far yet...I still get Duckbumps!

#### Cut Chemist

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##### Re: Question of the Week - Old Version
« Reply #347 on: 17/05/2006 05:40:49 »
How fast is warp speed??

#### JimBob

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##### Re: Question of the Week - Old Version
« Reply #348 on: 21/05/2006 03:38:22 »
Only Scotty and Sulu (and Spock) know. It isn't scientific, it is scince fiction, believed to be fasteer than the speed of light.

The mind is like a parachute. It works best when open.  -- A. Einstein

#### chris

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##### Re: Question of the Week - Old Version
« Reply #349 on: 16/06/2006 21:56:58 »
Now here's a question that I think you'll have fun with:

"Why does a mirror reverse things in the horizontal, but not the vertical axis?"

Chris

"I never forget a face, but in your case I'll make an exception"
- Groucho Marx

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##### Re: Question of the Week - Old Version
« Reply #349 on: 16/06/2006 21:56:58 »