So I was reading this old internet story about the Thunderwell Project in which a manhole cover has been claimed by some to have been the first man-made object in space.
Whilst this is an interesting story, I don’t think we’re going to get anything more than guesses regarding whether the man hole cover ever actually escaped the Earth’s gravity. I’m very dubious but feel free to chime in with any further thoughts about it.
However I am left wondering about how one might measure the speed of the cover as it exploded upwards. You see the cover was apparently captured flying upwards in a single frame of the film. I can’t find this footage but presuming that if one knows the speed of the shutter on the camera, why couldn’t one measure the length of the blur of the moving object to roughly calculate its velocity?
absolutely you could, but an accurate result you’d need to know much more than just the shutter speed, you would need to know the exact details of the camera design, shutter settings, frame rate of shooting, exposure and ISO. The records of this probably don’t exist for a speciality camera used during a top secret nuclear test 50 years ago.
Without that you’re only guessing and still even if you work out the instantaneous speed you don’t know whether the manhole cover vaporized into plasma a few microseconds after the picture was taken.
Thanks. I’m curious to know how one would go about working it out. Why would you need to know anything more than the shutter speed though? The ISO only affects how sensitive the film/sensor is. The image might be over or under-developed depending on the ISO but wouldn’t the size of the blur be the same regardless of the ISO? And I’m guessing the frame rate wouldn’t matter if you only had one frame.
In theory the ISO rating shouldn’t affect the length of the blurred image; except that if the image is under- or overexposed then it would be harder to find the start and end points of the blur. E.g. if it was overexposed then light from behind the theoretical start of the blur could wash out the image and reduce its apparent length.
Also you would need to know the object’s distance from the camera, and the focal length of the lens, so that you could work out what real-world length that streak represents. You’d have to make assumptions, too - e.g. that the object fired vertically upwards and there was no foreshortening.
You would also, in theory, need to know its size so you could take its apparent diameter (if it were stationary) into account - although if the object were moving that fast then presumably the rest diameter would be insignificantly small compared to the length of the blurred image.
Also, the opening and closing of a mechanical shutter are not instantaneous events. If the shutter swings to one side to open and then swings back in the opposite direction to close, then one side of the image will be exposed for longer than the other side; the difference in exposure times from one side of the image to the other will depend on the velocity of the shutter blade when it’s opening or closing, along with how far it has to travel.
I don’t know about the physics of manhole covers, but I am curious about what you said about the camera.
To measure the length of the blur you would either need a reliable reference object in the field of view, or know the focal length used for the photo and distance of the focal plane from the subject.
What do you mean by “shutter settings”? I suppose you might have to know how the shutter is designed (focal-plane shutter, one or two curtains, how they work at different shutter speeds, etc.), and the orientation of the camera (which should be easily derived from seeing the uncropped photo). For example, this photo shows distortion that can occur for a moving object as a result of shutter design. But this is all design, not something that IME you can set.
However, if you know the shutter speed, I don’t know why you would need the frame rate, exposure, or ISO setting.
