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Thanks :)

- Thread starter Raza
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Thanks :)

- #2

Doc Al

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Look up "perspective": http://en.wikipedia.org/wiki/Perspective_%28visual%29" [Broken]

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Consider instead watching an airplane just before landing at an airport.

I'm guessing it is probably travelling at 150 mph at least. That's 220 feet per second. So depending on the size of the airplane, it should be travelling more than once its own length per second.

But when you look at it versus the background of sky and clouds, it sometimes appears to be moving much slower than that. Almost "hovering" in the sky sometimes.

Part of this would be explained if the airplane was flying at an angle toward or away from you. But I've even noticed it when the airplane is travelling perpendicular to my line of sight.

Optical illusion? Poor assessment of the facts on my part? Lack of stationary reference points behind the airplane???

Or is it something more fundamental with the spherical geometry of our field of vision, etc. ??

Paul

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You're absolutely right. A Jumbo 747 generally lands at between 150 and 200 mph depending on the specific airport and local weather conditions at the time. But it certainly looks a lot slower than that.Consider instead watching an airplane just before landing at an airport.I'm guessing it is probably travelling at 150 mph at least. That's 220 feet per second. So depending on the size of the airplane, it should be travelling more than once its own length per second.

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I still don't get it. Can somebody explain it to me more in dept?

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I think your first response from Doc Al was as close to the truth as you'll ever get. Perspective.I still don't get it. Can somebody explain it to me more in dept?

A distant object IS moving fast. just the angle that movement makes at your eye is smaller the further away the object is, so the angular speed is smaller. That's all you're saying really - the further away the object is from you, the smaller the angular speed is.

- #7

jtbell

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Just like the further away the object is from you, the smaller it appears to be. When you look up at the moon at night, you can cover it up with the tip of your finger, held out at arm's length. But the moon is really one heck of a lot bigger than your finger!the further away the object is from you, the smaller the angular speed is.

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Thank you, taxi, I got it. :)

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- #10

prasannapakkiam

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rcgldr

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A graph of y=arctan(1/x) will help explain this:

**http://jeffareid.net/misc/pa.jpg [Broken]**

A 747 aircraft appears to be slow, because our brains don't properly interpret the size of the aircraft, and mis-interpret the aircraft as being closer than it really is. Having a small jet fly side by side with the 747 helps reduce this affect. Experience watching aircraft reduces this effect.

A 747 aircraft appears to be slow, because our brains don't properly interpret the size of the aircraft, and mis-interpret the aircraft as being closer than it really is. Having a small jet fly side by side with the 747 helps reduce this affect. Experience watching aircraft reduces this effect.

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How it explains? What is x and y?A graph of y=arctan(1/x) will help explain this:

It will be greatly appreciated if you give a proof.

- #13

rcgldr

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On the graph, Y is the perceived angle in radians. X is the distance of the object in multiples of height.How it explains? What is x and y?

It will be greatly appreciated if you give a proof.

The perceived angle of the object is the angle determined by the ratio of height to distance. For my example, I set the viewer's eye level on the ground, and the perceived angle is relative to the horizon. This is arctan(height / distance). The graph shows the angle as radians, so the maximum angle perceived is pi/2 (vertically looking up at the top of the object as it passes directly by). For 0 distance, I used the limit as X approaches zero to get an angle of pi/2 (actually I used a 2 paramter arctan function, that takes x and y as seperate components, to eliminate division by zero for distance zero).

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russ_watters

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This is the cause of a large fraction of "credible" UFO sightings, including a famous one by the Mexican Air Force a while back that got months of discussion here before someone (not from here) figured it out.

The fact that

Perhaps the easiest (maybe the only) way to deal with this problem is to view all events from a top-down, stationary 3rd party perspective. You should at least know your own speed and using vectors, you can compute the "absolute" motion of another object with some relative motion information. The catch here is that if you don't know distance, you get a range of possible "absolute" distances vs speeds for a certain observed relative motion. For this particular case, a little bit of logic might have led someone to consider the possibility of the objects being stationary, or being on the surface, and using that information the problem could have been solved with ease.

This is a common navigation problem, solved with a "maneuvering board": http://navsci.berkeley.edu/ns12b/lectures.htm [Broken]

Solving the problem during acceleration (such as in a turn) is only really possible (without some heavy math) during times of constant acceleration.

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