| From: David | 9/08/99
9:26:12 |
| Subject: Car wheel spokes | post id:
29386 |
| Why is it when a car or bike
wheel spins the spokes start by spinning in the same direction as the
wheel and then at a certain speed they appear remain stationary then spin
backwards, although obviously this not?
| |
| From: Bill Sherwood | 9/08/99
10:07:52 |
| Subject: re: Car wheel spokes | post id:
29400 |
| I guess that'd be when you're
watching them on TV - I reckon it's due to the frame rate of the TV
co-inciding at certain point with the position of the spokes, so that
sometimes they seem to be going forwards, stationary, and sometimes
backwards. But if you're seeing this with your own eyes, then I guess you need a faster processor on the your head ... ;) | |
| From: BrettD | 9/08/99
11:52:38 |
| Subject: re: Car wheel spokes | post id:
29452 |
| this has been dicussed before.
some people see it only on tv/film and attribute it to frame
rate. some people also see it in reality so the frame rate theory doesnt hold up there. Dr Karl once mentioned that the eye/brain has whats called a 'flicker fusion rate' which could be VERY loosely likened to frame rate. perhaps this differs for people and causes weird effects. perhaps if the orientation of a spoke wheeled is in the same position every time the 'flicker fusion rate' is approached, the image is momentarily stationary. Increase/decrease the wheel speed and the image appears to spin backwards/forwards. Ive only ever seen it spin forward, then hold, then spin backward at about 40kmh or 90kmh. i cant find any other explanation on the web though.. | |
| From: MichaelT | 9/08/99
13:46:36 |
| Subject: re: Car wheel spokes | post id:
29495 |
| Of course, if viewed under
artificial lighting, you have to take into effect the 100hz flicker of the
lights. I do remember seeing a big capacitor blow up a few years ago, [Hint: Electrolytics are not supposed to be hooked up to AC!] and remember it in 3 basic 'still frames'. The first image was of a small dense squirt of smoke, the second of a larger (8" or so) cloud and the third of a big mass of cloud/capacitor. Was really bizarre, and quite an effective wake-up for the rest of the class :) I still don't know how my mind saw an instantaneous event as 3 really highly detailed still images. I wasn't even stoned :) Regards, MichaelT | |
| From: michael | 9/08/99
16:28:09 |
| Subject: re: Car wheel spokes | post id:
29564 |
| The whole mechanism behind TV and
film is that the frame rate of TV or film is faster than the rate at which
the eyes and brain update perceived images. This means that what is
basically a series of still images is interpreted by our brains as "moving
pictures". Therefore it is not inconceivable that the wheel could be
spinning at a speed which matched the "refresh rate" of our eyes and
brains so we would see a still image. | |
| From: Daryn Voss (Avatar) | 9/08/99
17:16:15 |
| Subject: re: Car wheel spokes | post id:
29587 |
| Y'know, I really think anyone who
sees this in continuous light has some kind of brain trouble. When I have
heaps of resources, I shall investigate
further. | |
| From: Terry Frankcombe (Avatar) | 9/08/99
17:20:41 |
| Subject: re: Car wheel spokes | post id:
29590 |
| Y'know, I
really think anyone who sees this in continuous light has some kind of
brain trouble. When I have heaps of resources, I shall investigate
further. I too used to think this way, until I observed said reversals in a spinning top. I didn't take too much notice of the lights in the room, but they weren't standard fluros. | |
| From: Daryn Voss (Avatar) | 9/08/99
17:36:25 |
| Subject: re: Car wheel spokes | post id:
29600 |
| They don't have to be fluoros.
Incandescent light bulbs flicker with the A/c as well. If I ever see it in the sunshine, I'll have myself checked out. | |
| From: Megan...42 | 9/08/99
17:38:48 |
| Subject: re: Car wheel spokes | post id:
29602 |
| I've seen the effect in broad
daylight with car wheels - I can't speak for my brain, but my eyesight is
fine. | |
| From: MichaelT | 9/08/99
19:18:31 |
| Subject: re: Car wheel spokes | post id:
29625 |
| Incandescent lamps flickering?
All my electrical training and knowledge indicates
otherwise. Incandescent lamps work by pushing so much electricity through a wire it glows red/white hot. (sorry to state the obvious) I just did a quick test and it takes about a quarter of a second for a light bulb to stop glowing when it is turned off. With a 50hz supply, the bulb would not be flickering in any noticable manner. A flouro on the other hand works on an atomic scale. When the AC current crosses the zero point, nearly all glowing activity stops for a very brief moment. | |
| From: Daryn Voss (Avatar) | 10/08/99
3:22:01 |
| Subject: re: Car wheel spokes | post id:
29638 |
| Okay, now, does anyone here have
a turntable? My parents used to have one that had two sets of markings on
it. One of these was reinforced at 33 and a third revs/minute, the other
set was reinforced at 45 revs/minute. They worked, even though we didn't
have fluoro light in the house. (I don't think it was due to the refresh rate in my retinae: surely this (a) would vary at least a little bit over time and (b) would vary between individuals.) If the light level takes 250ms to drop to zero, then are you aure we'd expect no preceptible drop in light level during the quarter-time of the cycle (ie 5ms)? I'm going to have to find a turntable from somewhere. If anyone else out there has one with these markings, please operate it in incandescent light and posts the results here. 8^) | |
| From: steve(primus) | 10/08/99
3:39:11 |
| Subject: re: Car wheel spokes | post id:
29639 |
| Just a minute Daryn while I put
away my zimmer frame. We used to have one of those devices as well, this
one measured the speed for 78s as well. There used to be a little knob on
the old gramophones that adjusted the speed of the turntable. The device
was a disc with a series of black and white lines about 1mm thick and
about 2cm long around the central hole. You would put the disc on the
turntable and view it under normal 60 cycle per second electric light -
incandescent light globes (fluoros were unknown in house lighting in those
days). The black and white lines would appear as a series of greyish bars
that rotated clockwise if the turntable was too fast and anticlockwise if
the turntable was too slow. You would adjust the knob until the bars
stopped moving. It certainly worked in incandescent
lighting. | |
| From: Daryn Voss (Avatar) | 10/08/99
4:34:08 |
| Subject: re: Car wheel spokes | post id:
29641 |
| Hang on, I almost choked on my
prune juice. I think that this is confirmation that the variation in light
level under incand globes is perceptible. I guess this isn't proof that it is always the flickering that causes it, but I haven't ever seen this effect in sunlight. Maybe it's me that needs checking out. | |
| From: MichaelT | 10/08/99
8:11:36 |
| Subject: re: Car wheel spokes | post id:
29646 |
I had one of those turntables - they also had their own greenish flourescent light source to light up the lines. (BTW, we use 50hz in Australia now days :) I might drag a bicycle wheel up here and see if I can see any 'reversal' effect under an incandescent only lit room. It may have to wait till tonight, our house is too well lit. Regards, MichaelT | |