OT: Physics question, calculate speed of a moving car

[Pherdinand]

wyk, you are a good example of a student that works with numbers that are totally off and still the final result is amazingly close to the good one🙂)

 

[ClaremontPhoto]

How about these racing cyclists?

I think 1/15 or 1/30 from about 4 metres. They look very fast to me, faster than the car...

 

[jamiewakeham]

Quite... using red = 650nm and green = 510nm, you come up with about 1/3 the speed of light, using (because if I don't, I'll fry my poor little sixth-former's brains) the non-relativistic shift equation.

Peter - we really shouldn't get into the quantum energy thing here...

Jon - I'll leave this as an exercise for the student 🙂

Must get back to teaching these kids in front of me!

Cheers
Jamie

 

[JohnL]

If those are brightness variations and if they are caused by the street lighting at 50Hz, and if they are about 100mm (4") apart (eyeball guess), then the speed is 18km/hr. Less than the other estimate, but same order of magnitude.
Maybe the shutter speed was slower than 1/30? The fact that there seem to be about 5 ghost images suggests that the shutter speed might have been about 1/10 (say 1/8). That would bring the other estimate into fairly close agreement with this one.
Feel free to disagree with the premises.

 

[jamiewakeham]

No, that seems a pretty good argument, John. Of course, without actually knowing what the shutter speed was, we're just picking numbers out of a hat (unless we decide that those are variations in brightness due to 50Hz mains, in which case we can use them as a clock rather than the shutter).

I'm not yet convinced that they are 50Hz fluctuations, mind you, mainly because they imply a shutter speed of about 1/8, and because the building in the background is very sharp for 1/8 handheld! I suppose it could have been on a tripod, but that doesn't seem very likely to me. I'm off to see the rest of the physics department...

Cheers
Jamie

 

[pvdhaar]

..

Peter - we really shouldn't get into the quantum energy thing here...

Fully agreed Jamie, we'd better leave this as an exercise for the teacher 😛

Just having fun of course 😀

 

[jamiewakeham]

D'oh. Silly me. That's what I get for writing this and explaining the nuclear reactions that power the main sequence of stellar evolution at the same time...

There are two bright periods per full wave of AC current - one in the forward direction, on in the backward. Therefore 50Hz current has 100 'pulses' per second. So five pulses would imply a shutter speed of 1/20 (is there such a speed on this camera? Could be six pulses, implying 1/25?) and hence a velocity of about 10m/s or about 20mph.

And I could believe that this shot was handheld at 1/20. So yes, I believe that they are periodical fluctuations due to AC mains.

Cheers
Jamie

 

[jaapv]

No, that seems a pretty good argument, John. Of course, without actually knowing what the shutter speed was, we're just picking numbers out of a hat (unless we decide that those are variations in brightness due to 50Hz mains, in which case we can use them as a clock rather than the shutter).

I'm not yet convinced that they are 50Hz fluctuations, mind you, mainly because they imply a shutter speed of about 1/8, and because the building in the background is very sharp for 1/8 handheld! I suppose it could have been on a tripod, but that doesn't seem very likely to me. I'm off to see the rest of the physics department...

Cheers
Jamie

Hi Jamie!

Handholding a RF at 1/8 is perfectly feasable. I get a succes rate of about 35% doing that using a 28 or even 35.

 

[JohnL]

Hi Jamie!

Handholding a RF at 1/8 is perfectly feasable. I get a succes rate of about 35% doing that using a 28 or even 35.

Furthermore, it could easily be a wide angle lens, which makes it even more possible - and he could have been propped against a wall or a lamp-post.

I was not aware that you get brightness peaks twice per cycle - I suppose that would be at 0 and 180 degrees phase angle - but if this is the case then then shutter speed (for 5 images) must have been slower than 5/100 = 1/20. I guess it could have been 1/15, which could even pick up traces of a sixth image. It's also possible the camera used had 1/2 or 1/3 stop shutter speeds - who knows? I guess we are now about as close as we are likely to get.

 

[jamiewakeham]

Sorry - my previous post should have read "Could be six pulses, implying 1/15?".

We can work out just a little more, John, by looking up details of the camera used. The Oly 35 RC has shutter speeds of 1/15, 1/30 and upwards. If we accept that there are either five or six images, and that they were occuring at 100Hz intervals (two per cycle, at [as you suggested] 180 degrees seperation) then the only possible shutter speed was 1/15.


Incidentally, the lens is 42mm. I have no doubt that Jaap can handhold a 35mm lens at 1/8 (I can't, but I'm not that good and haven't been practicing long enough 🙂) and thus it seems perfectly possible that the photographer could have handheld this at 1/15. Or, as you say, there could have been a handy wall or lamppost involved.

And with that, I'm off home to read my new PS Elements book (thank you, Jaap!).

Cheers
Jamie

 

[Little Prince]

Been away from RFF for a few days now, only to return and find ... what?!

He He,.. I have enjoyed catching up on this thread. What funny exercises people can come up with to amuse themselves! It's good to apply some physics, even elementary, to something we actually see in real life. Or is that an accurate description of what we see on the negative? I feel like a school kid 🙂. Not that I'm old or anything.

 

[chenick]

Wow!
What a response! Some more details:
It was handheld, I made a special effort to stay still but I was still surprised at how little blur there was. As far as I remember, the camera was on AE (CQ says the Oly SP has "a shutter lockup outside the AE range of 1/15th to 1/250th" which means the shutter should not be less than 1/15. (bearing in mind this is a mechanical shutter over 30 years old)

Still, interesting to note the figures are in the right ballpark!

Cheers!
Nick

 

[wyk_penguin]

wyk, you are a good example of a student that works with numbers that are totally off and still the final result is amazingly close to the good one🙂)

OMG, I think I have mixed up things.. I see... longer wavelength is red.. (keep on confusing them with frequencies :bang: ) :bang: but it is the ratio of the wavelengths that is important. 😀 Physics works, .... even for those who miswork it.