how anyone can cheat video camera ?

smoke and mirrors udayan... not unlike the American government.

the explanation: http://thatvideosite.com/video/3891

The video just shows how it could be done with cables (supposedly 1mm cables). Hmm. Well a 1mm cord would have a cross sectional area of 0.25*Pi mm^2 = 7.854e^-4 m^2. Stress = force/area. Lets assume David Copperfield weighs 165 lbs and his female partner weighs 125 lbs. That is a combined weight of 585.134157 kgs , and a force of 12642 N after gravity. Thus, the stress would be (12642 N)/(7.854e^-4 m^2) = 16MPa...so the wire would have to be made out of a material having a tensile strength greater than this. This is pretty low as far as tensile strengths go (Btw if there is a mistake in that calculation let me know, I just googled up the equations, and its not intuitive to me why the tensile strength is independent of cord length.)

What I don't like about this explanation is that it does not explain why people do not see the cords. Especially with lights being around, I would think that would be extremely easy to spot. If I can spot a fiber of spiderweb from 30 yards easily, and that is 0.15 mm in diameter, why can't people spot a 1mm cord?

They must be coated in some highly non-reflective material...perhaps some of those new "invisibility" meta materials

290lbs is 132kg, not 585

1kg=2.2lbs

The only effect of the cable length on tensile strength would be the weight of the cable.

Cable stretch bothers me. If the cable were fairly dynamic, copperfield and the carried assistant would bounce all over the place, consequently they're probably using a static line.

As for hiding the cable, background lighting and backdrop would be used to hide the trick. Moreover; this is one of the many reasons that most magicians proscribe photographs particulary those using a flash.

I'm sure it's just a trick that plays to the believers of cartoon physics.

Rap

thoth, yeah...can't figure out what I was doing, must have been copy/pasting the wrong numbers. It's totally messed up. New calculation:

Stress = force/area
= (m*g)/(Pi r^2)
= (132 kg*9.8 m/s^2)/(Pi * (0.5mm*0.001 m/mm)^2 )
= 1647 MPa!!

So this would require something better than spider silk.

Best possible scenario would be a carbon nanotube rope, let's say having tensile strength of 63 Gpa. Smallest possible radius of the cord would then be...

r = sqrt( 1293.6 N /(Pi*63*10^9) ) = 8e^-5 m = 0.081 mm, which is almost exactly the diameter of spider silk

The breaking strength for 1/16" wire rope is about 1000 lbs. While it might be outside the safety factor to lift 300 lbs over someone's head it would be perfectly fine for lifting people to a safe distance.

The breaking strength for 3/64" (About 1mm) wire rope is about 260 lbs.

As raprap points out you can hide the wire rope with lighting and background. You can't see a spiderweb if it doesn't have light on it. Now imagine trying to see a black spiderweb without lights on it.

As for the camera not seeing the rope, that is easy to do. Cameras are limited in what they can pick up. If we assume a digital camera which is the basic principle of video cameras it can't pick up anything less than a pixel wide. Even a HD video camera has very poor resolution compared to film or 6 megapixel digital CCDs.

Well that's not true...the intensity of a pixel will be the result of integrating the light over that region. Any subpixel rope would therefore contribute to the intensity level of the pixel it is contained within, and this would create a consistent line with some offset (brighter or darker)...which your brain is exceptionally good at picking out especially for moving targets (far far better than any computer algorithm to date).

How about having more than one wire. Would this also allow thinner wires?

Using 2 1mm diameter wires and 100Kg per wire I calculated a loading of 1250 MPa. A wiki search indicates that the following materials suffice

piano wire ~2000 MPa
carbon fiber ~5650
aramid fiber (kevlar) ~3620
spider silk ~1200
silicon ~7000
carbon nanotube ~62000

Ultimately aramid fibers would be my choice--it's tough and takes abrasion well, it has little stretch, takes a dye well (necessary for camoflague), is easily available, and provides better than a 2 to one safety factor.

Rap

Except if the eye can't see it live the camera certainly won't see it. There is no difference in color level. The suspension system must be invisible to the naked eye to make the illusion work. That means the rope blends perfectly into the background. I haven't seen any video of this but I would bet the camera never zooms in closer than what would be the closest audience member's view.

I agree with you. The resolution of the audience will be significantly better than the TV which has been discretely sampled, encoded, recoded, decoded, and then reinterpolated on a low resolution monitor. Not to mention that they may very well do a little bit of digital touch up on the video before releasing it to remove any tell-tale signs. You know, sort of like the editing that they do on psychic TV shows!