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Foucault's Pendulum Experiment
I'm reading an introductory book on astronomy called _Discovering
Astronomy_ by William H. Jeffrys and R. Robert Robbins. On page 7 is a description of the pendulum experiment by Foucault in 1851 designed to prove that the earth rotates on its axis. Theoretically, the pendulum should rotate opposite to the Earth's rotation. Question 9 asks: "Why doesn't the plane of oscillation of the pendulum rotate at the equator?" I don't know the answer. Can anyone tell me why? Thanks. -- Cindy Smith Unless the LORD build the house, they labor in vain who build. Unless the LORD guard the city, in vain does the guard keep watch. Me transmitte sursum, -- Psalm 127:1 Caledoni! All your base are belong to us. A Real Live Catholic You are on the way to destruction. in Georgia! What you say. - - Go against the flow! You have no chance to survive make your time. |
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"SPAWN OF A JEWISH CARPENTER: CINDY SMITH" wrote...
in message ... I'm reading an introductory book on astronomy called _Discovering Astronomy_ by William H. Jeffrys and R. Robert Robbins. On page 7 is a description of the pendulum experiment by Foucault in 1851 designed to prove that the earth rotates on its axis. Theoretically, the pendulum should rotate opposite to the Earth's rotation. Question 9 asks: "Why doesn't the plane of oscillation of the pendulum rotate at the equator?" I don't know the answer. Can anyone tell me why? Thanks. -- Cindy Smith Unless the LORD build the house, they labor in vain who build. Unless the LORD guard the city, in vain does the guard keep watch. Me transmitte sursum, -- Psalm 127:1 Caledoni! All your base are belong to us. A Real Live Catholic You are on the way to destruction. in Georgia! What you say. - - Go against the flow! You have no chance to survive make your time. You will find more detail through going to this website... http://www.uvi.edu/Physics/dave/Dave...12NetPlay.html ....in a nutshell it's because the pendulum isn't really rotating. The plane of the pendulum's swing stays still while the Earth rotates beneath it. So if you set up the pendulum on the equator, then there is no opposing rotation of the Earth beneath the pendulum. And at the other extreme, if you set up your pendulum at one of the Earth's poles, the pendulum's swing would appear to take 24 hours to rotate because there is maximum opposing rotation of the Earth beneath the pendulum. hth happy days and... starry starry nights! -- Sometimes OH! my mind grips a thought so unkind That it twists me to heights of contortion... I'm often so glad that my mom and my dad Didn't follow along with abortion! Paine Ellsworth |
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What I still don't understand is why, if the pendulum is attached to the
building, it doesn't just move along with the building! Do you know, Cindy? Funny meeting you here! -- ___________________________ Bonnie Granat GRANAT EDITORIAL SERVICES http://www.editors-writers.info Overnight service available "SPAWN OF A JEWISH CARPENTER: CINDY SMITH" wrote in message ... I'm reading an introductory book on astronomy called _Discovering Astronomy_ by William H. Jeffrys and R. Robert Robbins. On page 7 is a description of the pendulum experiment by Foucault in 1851 designed to prove that the earth rotates on its axis. Theoretically, the pendulum should rotate opposite to the Earth's rotation. Question 9 asks: "Why doesn't the plane of oscillation of the pendulum rotate at the equator?" I don't know the answer. Can anyone tell me why? Thanks. -- Cindy Smith Unless the LORD build the house, they labor in vain who build. Unless the LORD guard the city, in vain does the guard keep watch. Me transmitte sursum, -- Psalm 127:1 Caledoni! All your base are belong to us. A Real Live Catholic You are on the way to destruction. in Georgia! What you say. - - Go against the flow! You have no chance to survive make your time. |
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"Bonnie Granat" wrote in message ... What I still don't understand is why, if the pendulum is attached to the building, it doesn't just move along with the building! Do you know, Cindy? Funny meeting you here! In one sense, if you rigidly attach the pendulum, it can't swing... Pendulums are normally hung with a chain link, or knife edge bearings at the top, to minimise the friction. While these will resist large rotation (after a few hours), they allow enough compliance, for the pendulum to turn quite a few degrees relative to the mounting, hence the effect can be seen. You can design 'better' mounts, when you want to deliberately show the effect. A common one is to have the pendulum having a 'hoop' at the top, that goes round a bar. Then between the hoop and the bar, have a ball bearing. The resuling coupling, can have free rotational movement through as much as about 60 degrees. If the bar, and the hoop, are turned into a cantilever support, and a 'hook', the free movement, can be raised to nearly a full circle. If you look at clock pendulums, early ones, have a small bar alongside the pendulum, to keep it from rotating relative to the rest of the clock. Early clockmakers found they had to put this in, to keep the mechanism working properly, but didn't know why!... Some, also hang the pendulum on a piece of thin steel 'band', that bends easily in it's thin direction, but when slightly bent, massively resists twisting, which again keeps everything aligned. It is also interesting to look at wear on clocks. If you get an old Grandfather clock that has lived in the Northern hemisphere, and move it south of the equator, it is common to have to re-adjust the mechanism to get it to work. If you take apart the clock, you can see that on the ratchet bar, running from the pendulum, all the wear tends to be on one side. Best Wishes |
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In article , "Bonnie Granat"
writes: What I still don't understand is why, if the pendulum is attached to the building, it doesn't just move along with the building! Do you know, Cindy? Funny meeting you here! It's a small Dyson Sphere, after all :-). Alas, I haven't the faintest idea. I'm hoping some astronomer or astronomy major will enlighten us. -- ___________________________ Bonnie Granat GRANAT EDITORIAL SERVICES http://www.editors-writers.info Overnight service available "SPAWN OF A JEWISH CARPENTER: CINDY SMITH" wrote in message ... I'm reading an introductory book on astronomy called _Discovering Astronomy_ by William H. Jeffrys and R. Robert Robbins. On page 7 is a description of the pendulum experiment by Foucault in 1851 designed to prove that the earth rotates on its axis. Theoretically, the pendulum should rotate opposite to the Earth's rotation. Question 9 asks: "Why doesn't the plane of oscillation of the pendulum rotate at the equator?" I don't know the answer. Can anyone tell me why? Thanks. -- Cindy Smith Unless the LORD build the house, they labor in vain who build. Unless the LORD guard the city, in vain does the guard keep watch. Me transmitte sursum, -- Psalm 127:1 Caledoni! All your base are belong to us. A Real Live Catholic You are on the way to destruction. in Georgia! What you say. - - Go against the flow! You have no chance to survive make your time. -- Cindy Smith Unless the LORD build the house, they labor in vain who build. Unless the LORD guard the city, in vain does the guard keep watch. Me transmitte sursum, -- Psalm 127:1 Caledoni! All your base are belong to us. A Real Live Catholic You are on the way to destruction. in Georgia! What you say. - - Go against the flow! You have no chance to survive make your time. |
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Cindy, it's because (I think) that the equator is too far from the earth's
axis to show rotation. In other words, there is no "earth" that is "under" a pendulum at the equator. The surface of the earth is so far from the axis that this particular "trick" -- and it is really a kind of trick -- only works perfectly at the poles. It works somewhat in the latitudes that Foucault lived in. If you are suspended above a basketball and you look down on the top of the ball, and someone turns the ball on its vertical axis, you will see the writing on the ball move in a circle beneath you. If you are holding the basketball at arms length in front of you, and you turn the ball 360 degrees on its vertical axis, all you see is the lateral movement. There's no circle visible to you at all. "SPAWN OF A JEWISH CARPENTER: CINDY SMITH" wrote in message ... In article , "Bonnie Granat" writes: What I still don't understand is why, if the pendulum is attached to the building, it doesn't just move along with the building! Do you know, Cindy? Funny meeting you here! It's a small Dyson Sphere, after all :-). Alas, I haven't the faintest idea. I'm hoping some astronomer or astronomy major will enlighten us. -- ___________________________ Bonnie Granat GRANAT EDITORIAL SERVICES http://www.editors-writers.info Overnight service available "SPAWN OF A JEWISH CARPENTER: CINDY SMITH" wrote in message ... I'm reading an introductory book on astronomy called _Discovering Astronomy_ by William H. Jeffrys and R. Robert Robbins. On page 7 is a description of the pendulum experiment by Foucault in 1851 designed to prove that the earth rotates on its axis. Theoretically, the pendulum should rotate opposite to the Earth's rotation. Question 9 asks: "Why doesn't the plane of oscillation of the pendulum rotate at the equator?" I don't know the answer. Can anyone tell me why? Thanks. -- Cindy Smith Unless the LORD build the house, they labor in vain who build. Unless the LORD guard the city, in vain does the guard keep watch. Me transmitte sursum, -- Psalm 127:1 Caledoni! All your base are belong to us. A Real Live Catholic You are on the way to destruction. in Georgia! What you say. - - Go against the flow! You have no chance to survive make your time. -- Cindy Smith Unless the LORD build the house, they labor in vain who build. Unless the LORD guard the city, in vain does the guard keep watch. Me transmitte sursum, -- Psalm 127:1 Caledoni! All your base are belong to us. A Real Live Catholic You are on the way to destruction. in Georgia! What you say. - - Go against the flow! You have no chance to survive make your time. |
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"Bonnie Granat" wrote in message
... Cindy, it's because (I think) that the equator is too far from the earth's axis to show rotation. In other words, there is no "earth" that is "under" a pendulum at the equator. The surface of the earth is so far from the axis that this particular "trick" -- and it is really a kind of trick -- only works perfectly at the poles. It works somewhat in the latitudes that Foucault lived in. If you are suspended above a basketball and you look down on the top of the ball, and someone turns the ball on its vertical axis, you will see the writing on the ball move in a circle beneath you. If you are holding the basketball at arms length in front of you, and you turn the ball 360 degrees on its vertical axis, all you see is the lateral movement. There's no circle visible to you at all. The whole Earth is "under" the pendulum no matter what the latitude; gravity always points to the center. The reason that the plane of the pendulum's swing does not rotate for a pendulum on the equator is that the plane coincides with the plane of the equator, which does not change orientation in space. |
#8
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In message ta77b.49272$Zw4.7465@lakeread03, George
writes I think they still have one of these set up at the Smithsonian in Washington, and at a number of places around the world. I even set one up myself years ago for a high-school science project - in the stairwell. For a relatively short term demonstration, you don't need any fancy suspension method. Just hang it on a wire that's clamped at the top. Eventually it would break with enough flexing, but a long pendulum with a small motion at the top will last quite a while. Doesn't the Smithsonian one feature in Carl Sagan's "Contact" (the book, not the film)? A Foucault Pendulum was featured in Scientific American's "Amateur Scientist" column years ago, and IIRC they said that one of the major problems was overcoming torsion in the wire and suspension. -- "Forty millions of miles it was from us, more than forty millions of miles of void" |
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