Rotating Pendulums

Pierre in Paris ( 48. 9 N , 2. 3 E ) (48.9 ^\circ \text{ N}, 2.3 ^\circ \text{ E}) and Sadie in Sydney ( 33. 9 S , 151. 2 E ) (33.9 ^\circ \text{ S}, 151.2 ^\circ \text{ E}) each perform the following experiment:

  • Set a pendulum swinging exactly along the East-West direction.
  • As Earth rotates about its axis, the plane of oscillation of the pendulum slowly rotates. Eventually, the pendulum will be momentarily swinging along the North-South direction.
  • Measure the amount of time it takes for this to happen for the first time.

Whose pendulum will take a greater amount of time?

Pierre Sadie Both take the same amount of time

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6 solutions

Carlo Wood
Nov 4, 2018

My reasoning was that if a pendulum would be close to the Northpole then the rotation of the earth would have no influence on it, it would just swing in the same plane - so, after 6 hours it would be swinging in the North-South direction. I think the sun comes up in the East (doesn't really matter), so looking on top of the northpole the Earth rotates towards the East or counter-clockwise and for an observer the plane in which the pendulum would swing would seem to rotate clockwise. On the South pole things are the same but the observer would be upside down and think that the pendulum rotates counter-clockwise (if you look on top of the southpole then the Earth rotates towards the East and thus clockwise). Therefore, my reasoning was - if you move the pendulum from northpole to southpole it has to change rotation from counter clockwise to clockwise, which is only possible when half way it doesn't rotate at all. So it seemed logical to assume that close to the equator it would rotate slower :*). Yay! No formulas!

nice one Carlo!

I worked it out without formulas also, but hadn't thought of your neat continuity argument.

Peter Macgregor - 2 years, 7 months ago

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Thank you! :smile:

Carlo Wood - 2 years, 7 months ago

I'm not sure I quite follow your argument about a pendulum at the North pole. For one thing, there is no East-West direction at the North pole, nor a North-South direction. What is true is that whatever the plane you start the pendulum swinging, after 6 hours the new plane will be at right angles to the original plane. (From the perspective of the observer. Actually the plane will stay the same and the ground underneath will rotate by 90 degrees.)

Paul Cockburn - 2 years, 7 months ago

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Not exactly on the north/south pole, but close to it. I consider(ed) the east-west direction locally to be always perpendicular to the north-south direction. But yes, the ground rotates under the swing-plane at the poles.

Carlo Wood - 2 years, 7 months ago

Surely it is always dependant on viewpoint and gravitational alterations for Einstein's concept of spacetime.

David Melville - 2 years, 7 months ago

Which is the North-South direction when you are on top of the Northpole? Hahaha

Pau Mazcuñán - 2 years, 7 months ago
Laszlo Mihaly
Oct 31, 2018

This problem is about the so called "Foucault pendulum". The angular velocity of the rotation of the pendulum's plane of motion is

ω = 36 0 sin ϕ / d a y \omega= 360^{\circ} \sin \phi /day

where ϕ \phi is the latitude. See more here: https://en.wikipedia.org/wiki/Foucault_pendulum

Sriram L
Nov 6, 2018

G force at the pole is generally lesser at /near poles than compared to other parts of earth. Also, time period of a standard pendulum varies inversely with the square root of g. Using these simple facts one can come to the answer.

The value of g only affects the period. The rotation of the plane of oscillation is not affected by the period of the pendulum only by the latitude.

Michael McLaughlin - 2 years, 7 months ago

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G Force at the poles is about 0.2% higher than average, not lesser!

Sebastian Meyer - 2 years, 7 months ago

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Not correct if viewing from a different point in spacetime.

David Melville - 2 years, 7 months ago
David Lu
Nov 10, 2018

the Earth rotates around its axis which means that if you're standing directly on the north or south pole, you're only rotating with the Earth's spin while if you're on the equator, you're not rotating at all and instead you're moving with the Earth's spin, what I just described were the two extremes on how you move while standing on the Earth. If you're on the equator and you're conducting this experiment then the pendulum will never rotate because you're not rotating at all but as you travel more north or south, it will start to increasingly rotate with the Earth thus making it possible to mark a point in time where you can see the pendulum swinging along the North-South direction.

Steven Adler
Jun 29, 2020

Time to turn is inversely proportional to the sin of the latitude. At the poles the direction of a Foucault pendulum swings a full circle in one day. At the equator, it never changes direction.

Vivek Sharma
Nov 10, 2018

In simplified terms, the period of rotation , T ( in hours) of the plane of oscillation for a Foucault pendulum is T = 24/ sin ϕ where ϕ represents the latitude of the place Here for Pierre , T = 24 /sin48.9° = 24/0.7535 = 31.85 hours

For Sydney , T= 24/ sin33.9° = 24/ 0.5577hours = 43.03 hours Longitude does not have much bearing on the oscillation time

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