Thread: A thought experiment involvng a large bridge

1. Originally Posted by Sticks
I also posted this on a non science based forum, to see what the response would be, and the post right after my OP got it spot on, with the reasons included and even brought up that the ball would behave like a pendulum

How does this rate as a thought experiment for getting people to think about physics?

I have absolutely no background in physics, but I found it an intriguing question and even got it right (if I understand all of the subsequent posts, of which there is no guarantee!).

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Jan 2009
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Originally Posted by NEOWatcher
You'd see a sagging deck, but it takes almost 2 miles for the deck to sag 1 foot.
I disagree. The bridge has been postulated as being perfectly flat. Photons travelling from distant parts of the bridge to the viewer's eyes would not be deflected far enough by the earth's gravity to change the bridge's appearance. It would appear perfectly flat; the lines along the edges of the bridge deck would appear perfectly straight and would recede toward infinity.

Since the bridge (as postulated) would be extremely long, it's also reasonable to suppose that the lines would converge so far that the human eye could not separate them, meaning the bridge would actually recede to infinity (visually.)

Originally Posted by publiusr
Haze would degrade the view anyway
Since the bridge deck, at least at the towers, would be above most of the Earth's atmosphere, that might not be a problem. If the height were such that the midpoint of the deck were tangent to the planet's surface, the view would appear to recede to the surface, meeting the Earth at the horizon. As I mentioned above, if it were long enough it could be reduced to a point (visually) before it got there. Of course, that would depend on the width of the deck. If the bridge were higher than that (tangential to the surface) it might be entirely above the hazy atmosphere.
Last edited by Gandalf223; 2009-May-22 at 05:01 PM. Reason: Completing my thought...

3. Originally Posted by Gandalf223
I disagree. The bridge has been postulated as being perfectly flat...
Yes and no. I was speaking from the casual observers notion of the surface of the Earth being interpreted as flat. So; Yes, sag is an unfortunate word, but I was trying to get some concept of the middle of the bridge appearing closer to the Earth's surface than the ends.

So; let me clarify that my comment is only perception and could probably be that way due to optical illusion effects, and is totally dependent on the ability to percieve the curvature of the Earth.

4. So, allow me to add another initial condition, since I think the original problem is quite nicely handled.

Now, take the bowling ball, still on the frictionless surface, and give it a very strong spin, say in a clockwise direction, at 75% of the angular rotation that it can withstand due to centripital accileration, with the spin axis (originally) pointed straight through the center of the earth.

What path will the ball take?

5. Originally Posted by jj_0001
So, allow me to add another initial condition, since I think the original problem is quite nicely handled.

Now, take the bowling ball, still on the frictionless surface, and give it a very strong spin, say in a clockwise direction, at 75% of the angular rotation that it can withstand due to centripital accileration, with the spin axis (originally) pointed straight through the center of the earth.

What path will the ball take?
If there is no friction between the ball and the bridge, the spin will have no effect on the ball's trajectory. The spin axis will remain pointing in the same direction in space, so it will move away from Earth's center as the ball moves over the surface, and even more so as the bridge turns with the Earth's rotation.

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