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Our Foucault pendulum rotated the wrong direction - why?

Oystein

Penultimate Amazing
Oystein
Joined
Dec 9, 2009
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This is largely a re-post from the February Stundie nominations in the Conspiracy Theories subforum. There, it's just an anecdote to go along with the topic of a nomination. Here, I am looking for the explanation.

So who can help me out here?

A little true story:

We had finished the curriculum for the semester in high school physics early, so as a reward we spent the last lesson (90 minutes plus a little extra) building a Foucault pendulum.
We let it swing for maybe 45 minutes, and indeed it had rotated somewhat during the time. We didn't measure exactly, but what we saw was probably close to the, say, 10-12 degrees we expected, and everybody was happy. Except me. I asked the teacher quietly if the pendulum should not have rotated clockwise, and he whispered back "yes, but don't tell the others!"



Now: Why did our pendulum rotate the wrong direction? I don't know for sure. For the longest time I suspected torsion in the wire, movement of air in the room, or some effects by the gallow from which we hung it - I remember that we weren't able to fix it as stably as we wanted and that it moved a little with every swing. My favourite explanation however is that we started the pendulum incorrectly by moving the ball a couple of meters away from center, holding still a little, and then release. I am certain with gave it angular momentum that way in the direction of earth's movement, or maybe even a little more by accident. When the amplitude diminished, the angular momentum converted to higher angular velocity, making the pendulum rotated ahead of the planet. I think the right way to start a F.P. is to let it rest dead center and then knock it with a second ball, like billard.
Does this make sense? I read over at Wikipedia that "the traditional way to do this is to use a flame to burn through a thread which temporarily holds the bob in its starting position, thus avoiding unwanted sideways motion". Would the bob then not have the same angular momentum as the floor vertically below or roof vertically above, and keep moving in unison with those spots?
 
I'd have to guess experimental error - ie, what you suggest above re: swinging the bob by hand sounds most likely to me (disclaimer - physics makes me run screaming into the night)
 
Obviously you conducted your experiment concurrently with the events recorded in Isaiah 38:7-8. Why have you taken so long to post this??






:D
 
"or some effects by the gallow from which we hung it "

Was the gallows steady as a huge rock, small wiggles could cause the effect.

Also you start the pendulum at the edge of the circle I believe, were you outside the circle? This could amplify a slanted release.

ETA:
I think it did not run long enough. If it had enough angular momentum to go the wrong way, it would take a long time to disipate. There are many web pages that explain why the pendulim moves, but essentialy there is a difference in the relative velecity between the north end of the swing and the south end of the swing.

The motion varies with the sime of the lattidue I think, and if you look here:
http://en.wikipedia.org/wiki/Foucault_pendulum_vector_diagrams

For the 45° North pendulum with latitudinal swing (Figure 3B) the support point of the pendulum swing is moving along with the direction of rotation and the surface velocity vectors on either side of the swing are not balanced. The rotation of the Earth is observable in relation to the pendulum swing because a change in relationship to the surface occurs. On one side of the pendulum swing the surface velocity vectors are angled upwards and to the side but on the other side the vectors are angled downwards and to the opposite side. Since these vectors are not all in the same plane as the pendulum swing and are not balanced in the same direction there is a change in relationship between the surface and the plane of the pendulum swing.
Click to expand...

So if th erelease imparted a momentum in the oppsotie direction it will take time for the small vector changes to compensate slowing, stopping and reversing the pendulum's motion.
 
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Dancing David said:
"or some effects by the gallow from which we hung it "

Was the gallows steady as a huge rock, small wiggles could cause the effect.
Click to expand...

How so? How signifficant would that be?
We constructed the pendulum in the main building of our school, which features a 3-floors-high atrium, with classrooms and halls all around on the 3 floors. We attached a little construction featuring a horizontal steel beam, maybe 1.5 meters long, to the handrail on the upper floor, with a steel wire hooked to its end, and an iron ball of some kilograms hanging on the wire 5-6 meters below. When the pendulum swung, the beam wasn't totally steady, it moved back and forth in the direction of the pendulum by some millimeters, I'd guess.

Dancing David said:
Also you start the pendulum at the edge of the circle I believe, were you outside the circle? This could amplify a slanted release.
Click to expand...

I don't remember anymore how exactly we released the pendulum, but let's assume we were careful, but not entirely precise. What is the significance of "me" (or whoever released the pendulum) standing outside the circle? I'd say if you stand inside, you'll get hit in short order! :D


Basically what I am looking for is the several ways you could tamper with the experiment, and what their respective influence and probability of reversing the rotation is. In particular, can someone explain to me this thing about starting with the ball at rest relative to the floor on the edge of the circle and thus with an angular velocity equal to (?) that of the floor.
 
I'd vote for "some effects by the gallow from which we hung it". Pendulums that are truly free to move in any direction are quite difficult to build, not something I'd expect a bunch of schoolies to knock together in a hour and a half. You usually end up with a pendulum that has a preferred plane of motion, like a the set of swings in a child's playground. If you started it's motion outside that plane, it would tend to correct itself until it's movement was in the preferred plane. Though with a massive enough bob, long enough leader, and a fairly well-built bearing, that tendency could be made arbitrarily small.
 
"moved a little with every swing"
I guess that would do it, it's a very small effect after all.
 
Was the swing in a north-south line, or an east-west line?

An east-west line would minimize the Carriollous effect, allowing a small problem with the gallows to get larger.
 
Oystein said:
In particular, can someone explain to me this thing about starting with the ball at rest relative to the floor on the edge of the circle and thus with an angular velocity equal to (?) that of the floor.
Click to expand...

I don't entirely understand Foucault pendulums, but I don't think that starting one off with a bit of sideways motion would cause the line along which it swings to rotate, the way the Earth rotating under it does. It would just cause it to swing in a narrow ellipse, instead of a perfectly straight line.
 
The 2012 magnetic reversal is starting, and the cosmic waves are having a larger effect on the iron weight used for your pendulum. ;)

Oops, almost forgot to add Quantum and Nano too.
 
The most likely reason is that it wasn't started with a drop, but with a push, and the push was not exactly toward the lowest point (determined with a plumb-bob) of the arc.
 
Oystein said:
My favourite explanation however is that we started the pendulum incorrectly by moving the ball a couple of meters away from center, holding still a little, and then release. I am certain with gave it angular momentum that way in the direction of earth's movement, or maybe even a little more by accident.
Click to expand...

You can, if you like, entirely forget about the earth's movement and work in a frame where it is not rotating. In that frame there is a Coriolis force, a force proportional to mass and the cross product of the velocity with a fixed vector (the rotation axis of the earth). That's the force responsible for the precession of the plane of the pendulum.

When you released the pendulum, if you gave it a push that wasn't towards the center, as 69dodge said the pendulum would then move in an ellipse (at least approximately). With no friction, I think Coriolis force would simply cause the major and minor axes of that ellipse to precess - although I haven't checked. But I think that's the only thing giving it a push would do - it could not cause the pendulum to swing in a plane and that plane to precess the wrong way.

I suppose if you didn't notice that the motion was ellipsoidal you might make a measurement error that could make you think the precession was opposite to what it really was (you could measure the direction of motion along two different parts of the ellipse). Could that have been it? 10 degrees isn't all that much.
 
Little 10 Toes said:
You started it from the wrong side.
Click to expand...

Little 10 Toes said:
But on a serious note, what did you use to keep the pendulum swinging?
Click to expand...

Nothing - the pendulum was long, massive and dense enough to keep swing for however much time we had left after constructing it - 30 minutes? 45 minutes? Amplitude sure was dampened considerably, but by the end of the lesson it was still swinging fine.
 
Another point. Your profile says you're in Germany. If the experiment was conducted at 50 degrees north latitude, the pendulum should fully precess once every 1/sin(50)=1.31 days, or by 11.5 degrees per hour. But that's only 8.6 degrees per 45 minutes, not 10-12 as you said in the OP you expected.

ETA - and torsion in the wire could be important, depending on how exactly you built the pendulum.
 
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casebro said:
Was the swing in a north-south line, or an east-west line?

An east-west line would minimize the Carriollous effect, allowing a small problem with the gallows to get larger.
Click to expand...

Really? Than that's a good point - Looking at the building on Google Maps, and being fairly certain of which wall we started parallel to, the line was about 20° away from east-west.
 
rwguinn said:
The most likely reason is that it wasn't started with a drop, but with a push, and the push was not exactly toward the lowest point (determined with a plumb-bob) of the arc.
Click to expand...

We definitely started it with a drop, even though I don't remember the exact method. Could well be that our release mechanism (hand?) had a little push this or that way, by accident.
 
Oystein said:
Really? Than that's a good point - Looking at the building on Google Maps, and being fairly certain of which wall we started parallel to, the line was about 20° away from east-west.
Click to expand...

casebro has that backwards - east/west motion maximizes the force.
 
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