BS: Stagecoach wheels running backwards

Subject: BS: Stagecoach wheels running backwards
From: Bizibod
Date: 13 Jan 06 - 03:18 PM

I KNOW that they don't actually, but how is it that they look as though they do? A really simple (but truthful)explanation would be much appreciated,this has bothered me on and off for years.

Subject: RE: BS: Stagecoach wheels running backwards
From: MMario
Date: 13 Jan 06 - 03:20 PM

optical illusion - your eyes can only process so fast - and the spokes end up slightly behind where they were when you saw them last, so even though they are turning forward they appear to be turning backwards.

Subject: RE: BS: Stagecoach wheels running backwards
From: beardedbruce
Date: 13 Jan 06 - 03:25 PM

And the film is taking a limited number of frames per second- so when enough of the frames show the spokes "moving backwards" ( ie, at just under one complete revolution fron the previous frame) you can see the effect you are commenting on.


You can produce the same thing with a design on a record and a strobe.

Subject: RE: BS: Stagecoach wheels running backwards
From: Bizibod
Date: 13 Jan 06 - 03:28 PM

Got it! Thanks, feel much better now !

Subject: RE: BS: Stagecoach wheels running backwards
From: gnu
Date: 13 Jan 06 - 03:30 PM

Records, strobes... a black light and some maryjane... yeah, I see it too, man.

Subject: RE: BS: Stagecoach wheels running backwards
From: jimmyt
Date: 13 Jan 06 - 03:49 PM

hell, gnu, you don'e even need the records strobes and blacklight

Subject: RE: BS: Stagecoach wheels running backwards
From: Wolfgang
Date: 13 Jan 06 - 04:20 PM

The answers are incomplete for they do not explain why sometimes the wheels turn forward and sometimes backward and why that changes with the speed.

If an object (picture) is displayed at two different positions in space one shortly after the other then (depending upon time and distance) the object (picture) is perceived to have moved from one position to the other (thats's why we can see motion with 24 stable frames per second). The perceived movement is into the direction of the second presentation. If an object (picture) is first displayed in one position and then in two position opposite to the first position it is perceived to move to the second position nearer to the first one. With the spokes, there are two choices from one film frame to the next, clockwise is a spoke and counterclockwise as well. To which spoke will the eye perceive the 'old' spoke to have moved? To that nearer to the old position. If that is clockwise and the coach moves from left to right or if that is counterclockwise and the coach moves from right to left then a forward motion of the wheels will be seen. in the other two cases a backward motion will be seen.

One can also have the perception of motion both clockwise and counterclockwise motion though nothing at all changes in the picture:

Rotating snakes

"It must be real I have seen it with my own eyes".

Wolfgang

Subject: RE: BS: Stagecoach wheels running backwards
From: Rapparee
Date: 13 Jan 06 - 04:32 PM

You can also see this phenomena on a 16mm film projector that's running an empty reel. (Assuming you can find a 16mm film projector....)

Subject: RE: BS: Stagecoach wheels running backwards
From: Amos
Date: 13 Jan 06 - 04:45 PM

I know some people like that projector, actually.... :D


A

Subject: RE: BS: Stagecoach wheels running backwards
From: number 6
Date: 13 Jan 06 - 04:51 PM

Great thread Bizibod !!

Keeping my eyes on this one.

sIx

Subject: RE: BS: Stagecoach wheels running backwards
From: Rapparee
Date: 13 Jan 06 - 04:52 PM

Ever watch a computer screen or television screen that's been televised?

Subject: RE: BS: Stagecoach wheels running backwards
From: Wolfgang
Date: 13 Jan 06 - 05:29 PM

The answers are incomplete for they do not explain why sometimes the wheels turn forward and sometimes backward and why that changes with the speed.

If an object (picture) is displayed at two different positions in space one shortly after the other then (depending upon time and distance) the object (picture) is perceived to have moved from one position to the other (thats's why we can see motion with 24 stable frames per second). The perceived movement is into the direction of the second presentation. If an object (picture) is first displayed in one position and then in two position opposite to the first position it is perceived to move to the second position nearer to the first one. With the spokes, there are two choices from one film frame to the next, clockwise is a spoke and counterclockwise as well. To which spoke will the eye perceive the 'old' spoke to have moved? To that nearer to the old position. If that is clockwise and the coach moves from left to right or if that is counterclockwise and the coach moves from right to left then a forward motion of the wheels will be seen. in the other two cases a backward motion will be seen.

One can also have the perception of motion both clockwise and counterclockwise motion though nothing at all changes in the picture:

Rotating snakes

"It must be real I have seen it with my own eyes".

Wolfgang

Subject: RE: BS: Stagecoach wheels running backwards
From: Bill D
Date: 13 Jan 06 - 05:40 PM

*from memory* I think the phenomenon involved is called "precession"

Subject: RE: BS: Stagecoach wheels running backwards
From: Bizibod
Date: 13 Jan 06 - 06:27 PM

Oh my goodness! So what's going on with those snakes then? And why is it only the ones you're NOT looking at that are revolving?!

Subject: RE: BS: Stagecoach wheels running backwards
From: open mike
Date: 13 Jan 06 - 06:40 PM

how does the computer know which ones i am NOT looking at??!!

Subject: RE: BS: Stagecoach wheels running backwards
From: Peace
Date: 13 Jan 06 - 06:57 PM

"I KNOW that they don't actually, but how is it that they look as though they do?"

The simple explanation is this: The stagecoaches you saw were going backwards.

Subject: RE: BS: Stagecoach wheels running backwards
From: JohnInKansas
Date: 13 Jan 06 - 07:01 PM

The early "movie industry" spent lots of time and money looking for a way to make the stagecoach wheels go the right way, and eventually figured out that most people just accept it and it doesn't really matter "artistically."

The name usually applied to phenomena of this sort is "strobe effect." Many record turntables have/had a circle of bars marked on the periphery of the platter, with a small neon light illuminating them. Since neon on ac power actually extinguishes and relights every time the voltage goes through zero, you get a light that flashes at 2x the line frequency - 120 Hz for US 60 cycle, or 100 Hz for Brit 50 cycle power.

If the "next bar" has moved exactly to the position of the previous one when the light comes back on, you see a stationary pattern, and theoretically your platter is turning at proper speed.

If the "next bar" has moved past the where the previous one was, the perceived pattern will rotate in a forward direction, and the platter is turning too fast.

If the "next bar" doesn't quite make it to where the previous one was, the pattern will appear to rotate "backwards" - like the stagecoach wheels - and the turntable is going too slow.

In the case of the stagecoach, if you watch very closely as a coach in a movie starts up, you will see a "flash" instant where the spokes appear to stop rotating in the "correct direction," but the coach will appear to have twice as many spokes as are really there. This happens when a spoke moves exactly half-way to where the preceding spoke was, so that at the second succeeding frame it's also seen in the same place where the preceding spoke was two frames earlier. This is also the instant at which the "apparent rotation" first reverses and it begins to look like the wheel is turning backwards. For a typical rear stagecoach wheel 40 inches in diameter (variable) with 18 spokes (pretty standard) this will happen, for a 24 frame per second camera, at about 5 mph - a brisk waking speed (about 3 mph for a 36 frame/sec camera). As the coach speeds up, the "apparent rotation" will be first forward, then backward, then forward, then.... Unfortunately you have to double the speed to get from where the last "backward phase" starts to where the motion next reverses, so for "high speed runaways" you'd need a 90 mph stagecoach to see the wheels go the right way (or 8 foot diameter wheels).

For the worms that Wolfgang provided, the "stobe effect" is provided by what's called the "saccadic motion" of your eyeballs, which is another story. Suffice it to say that frogs can't see the worms move.

John

Subject: RE: BS: Stagecoach wheels running backwards
From: Dave the Gnome
Date: 14 Jan 06 - 09:39 AM

Flourescent lights provide a strobe effect as well as strobes 'proper' and movie cameras. If you get chance look at the wheels of passing vehicles next time you are on a well lit motorway.

For this very reason it was deemed not a good idea to have strip lighting in workshops running lathes, drills etc. Imagine a drill turining at 60000 rpm and a light flickering a thousand times a second in synchonisation? - Looks like the dril has stopped. And believe it or not it did happen more than once in the past with dire consequences:-(

Cheers

DtG

Subject: RE: BS: Stagecoach wheels running backwards
From: Severn
Date: 14 Jan 06 - 10:20 AM

What I always wanted to know, is why the rear wheel on the passenger side of a stagecoach invariably fell off in high speed chases. I assume this was solely Hollywood and that they weren't all really horse-drawn "Pintos" or "Vegas".

Hell, the folks back then would have shot or hung Ralph Nader two minutes after he finally arrived in town on whatever form of conveyance he took to reach there! They'd have killed him outright just to keep him from complaing that the rail they were to ride him out of town on was unsafe or that the tar used with the feathers was toxic. They didn't want to hear any famous last words.

Of course, if the gun misfired or the rope broke, he would have had the last laugh for about two seconds until replacements were found on the run.

Subject: RE: BS: Stagecoach wheels running backwards
From: Ebbie
Date: 14 Jan 06 - 08:58 PM

One of the illusions I like is the perception that one is moving in relation to a stationary object, when it is just the opposite.

About a mile from here on a hardpan road is a grove of big spruce trees hovering over a mountain stream. The water has undercut the bank and several of the trees are in danger of toppling into the river in some not so distant winter storm. I like to seat myself on one of the tree roots that rear above the water and feel myself moving swiftly upstream. I feel almost weightless, the ride is so silky and smooth. Come to think of it, that's the kind of thing I did when I was a kid - neat to be able to replicate an old experience.

Subject: RE: BS: Stagecoach wheels running backwards
From: Les from Hull
Date: 15 Jan 06 - 08:54 AM

Slight drift into music - years ago I could tune my bass guitar using the TV. The plucked E string would almost seem to stop by the light if the TV screen.

Subject: RE: BS: Stagecoach wheels running backwards
From: jonm
Date: 15 Jan 06 - 11:54 AM

Actually, all machine shops DO have fluorescent lighting because it doesn't flicker. Many hobbyists in their shed with 50Hz watchmaker's lathes and single overhead bulbs have injured themselves thinking the lathe is stationary when in reality it is rotating at an exact multiple of 100 cycles per second (common with single-phase lathe gearboxes).

Cine film runs at 24 frames per second, so if the wheel rotates a whole number of spokes in 1/24 sec. it will appear stationary. Slightly faster and it will appear to move forward very slowly. Eventually the forward motion will slow down and stop again as a larger multiple of spokes passes in exactly 1/24 sec.

TV film runs at 25 frames per second, whcih is why movies are always shorter on TV and why "made for TV" movies somehow look different in terms of clarity. So why is the wheel still stationary when the film is shown on TV?

Only joking!

Subject: RE: BS: Stagecoach wheels running backwards
From: MMario
Date: 15 Jan 06 - 03:17 PM

YOu can see this phenomenon in real life as well - fans, etc, depending on the speed of the blades.

Subject: RE: BS: Stagecoach wheels running backwards
From: JohnInKansas
Date: 15 Jan 06 - 03:44 PM

A single fluorescent tube will always flicker, and can be a real hazard in places with rotating machinery. Fluorescent lighting in workshops should always use two or more - always an even number - of tubes in each individual fixture. Since the early 1940s (I think?) in the US, ballasts for multitube fluorescent fixtures have been required by codes to incorporate a "lag" in the circuit for one side, so that the flicker in one tube is wiped out by the out of phase flicker in the other tube. Common lamp tubes also now use a more "persistent" fluorescent that continues to glow for a bit after it's excited, which helps to reduce the flicker effect.

Especially in workshops, when a tube nears end of life both tubes should always be replaced at the same time, since a difference in brightness between the two will let the older one, which usually has more pronounced flicker, "dominate" the impression of the viewer and produce a strobe effect that can be hazardous.

John

Subject: RE: BS: Stagecoach wheels running backwards
From: GUEST,leeneia
Date: 15 Jan 06 - 04:54 PM

That's good to know, John. I'm constantly amazed at the extent of your knowledge.

Subject: RE: BS: Stagecoach wheels running backwards
From: JohnInKansas
Date: 15 Jan 06 - 06:07 PM

Someone will be along soon to argue with the "A single fluorescent tube will always flicker" statement. There are some tubes that don't.

A fluorescent tube of the common kind operating on AC power in conventional manner will always flicker. As a general statement, any lamp that uses a "gas arc" or "plasma discharge" will flicker at line frequency (or usually at 2 x line frequency). This includes kinds frequently used for farmyard and street lighting, and accounts for a previous comment about wheels on passing vehicles showing the strobe effect by looking like they turn the wrong way.

Some large offices and industrial shops use fluorescent tubes of different construction that have little or no flicker. These will usually not work in typical home fixtures, and the tubes usually are distinctive in having only one "pin" on each end for connection. (And not all single pin tubes are of this kind.) There are also some, usually small, single-tube devices, of the sort one would use for a "spot lighting" application, that convert the AC power to rather high voltage DC or to a high frequency AC, and since the lamp itself is DC or high frequency AC they produce little or no discernable flicker.

A characteristic that limits the more widespread use of fluorescents in typical home applications is that they are not "dimable." The recent offering of "replacement fluorescents" that can be screwed into standard incandescent bulb sockets presents the likelihood that people may try to use them in lamps and/or fixtures with "dimmer switches" or rheostats. Aside from the fact that they won't work properly, there is a slight hazard that something will burn up - or at least burn out rapidly.

Dimmable fluorescents do exist, but in all cases I've seen they've been "special designs" using moderately high voltage at RF frequencies. At least one mid-size light commercial airplane I know of has used a system of this kind since the mid 1960s for interior lighting in the passenger compartment. In effect, it's more of a "fluorescent augmented neon sign" in operation. I haven't seen anything similar for home use, but they may be out there.

John

Subject: RE: BS: Stagecoach wheels running backwards
From: Liz the Squeak
Date: 15 Jan 06 - 06:11 PM

So if the stagecoach were going backwards really fast, would the wheels look like they were going forwards?

LTS

Subject: RE: BS: Stagecoach wheels running backwards
From: Bizibod
Date: 15 Jan 06 - 07:37 PM

D'you know, I believe they would!And the horses would look most peculiar.

Subject: RE: BS: Stagecoach wheels running backwards
From: JohnInKansas
Date: 16 Jan 06 - 01:02 AM

LTS -

One of the tricks an early movie producer is reported to have tried was to put the stagecoach on a rubber tired wagon, with the wagon hidden from view of course, and use an electric motor to rotate the coach wheels backward so they'd look like they were going the right way.

I think the report of this was from someone's imagination, although "inventors" will try to do lots of pretty bizarre things; but it is true that the wheels will look like they rotate in the direction opposite to their actual motion, regardless of which way they're actually going, assume the frame rate and wheel rotation rates are in the proper frequency ranges.

John

Subject: RE: BS: Stagecoach wheels running backwards
From: Liz the Squeak
Date: 16 Jan 06 - 03:50 AM

Yes.. I see your point... the horses would have great difficulty, as they don't like going backwards at the best of times.

LTS

Subject: RE: BS: Stagecoach wheels running backwards
From: The Fooles Troupe
Date: 16 Jan 06 - 05:13 AM

Reminds me of a movie short I saw years ago. Was a 'Western' spoof - all done without horses. They used stop motion for the guys galloping along - and even put horseshoe marks on the ground! The stage coach was dealt with by having a wheel-less box on the ground - no worries about wheels seeming to move backwards!

Subject: RE: BS: Stagecoach wheels running backwards
From: Paul Burke
Date: 16 Jan 06 - 06:53 AM

How could this thread have got so far without someone mentioning sampled data systems, the Nyquist frequency, and aliasing? No education the youth of today....

Subject: RE: BS: Stagecoach wheels running backwards
From: Mr Red
Date: 16 Jan 06 - 07:42 AM

Nyquist - oooooer I wuz just about to

Fer wot it's wurf
think "looking into endless mirrors". The sampling frequency effect is like a mirror at one point (Fs/2 actually) and there is one behind you (at zero frequency) and all multiples produce replicate patterns. Like the harmonics in the musical scale repeat a similar but not same pattern.

Now can anyone help me with a source of screened twisted tri 24AWG wire?

(I really must stop looking at BS threads at work)

Subject: RE: BS: Stagecoach wheels running backwards
From: Geoff the Duck
Date: 17 Jan 06 - 05:04 AM

Back wheels faling off coaches - if they made the front one fall off at speed it might roll in the wrong direction and tangle with one of the back wheels causing an accident. The front of the cart would also drop down and would probably inure the horses, cause the whole shebang to smash and damage the actors or stuntmen.
With the back wheel, it should be possible to balance the rig so that it looks good, but no parts of the carriage will actually touch the ground in an uncontrolled way. It should be a lot safer for the people who are on the coach.
Was Nyquist the bloke in the black hat? How frequently did he get shot then?
Rubber tired wagons - didn't they use them in Frankie and Johnne - "Bring out your rubber tyred hearses, bring out your rubber tyred hacks. There's seven men going to the graveyard, but only six men comin' back......."

Quack!
Geoff the Duck.

Subject: RE: BS: Stagecoach wheels running backwards
From: The Fooles Troupe
Date: 17 Jan 06 - 06:59 AM

Early rubber tyres were solid, the pneumatic ones were a later invention.