Pendulum and coupling to Earth's spin

[abskebabs]

During my physics tutorial session recently, I remember a girl asking a question that she said her dad asked her based on physics that seemed to make us step back a little and think. So I'm sure you may enjoy it:-)

 

Ok basically its kinda 2 questions. What is it that causes the Earth to spin, is it some remnant of angular momentum that still exists since its creation. or is there some other explanation?

 

Also Why is a pendulum that is stationed on the Earth's surface unnaffected by the Earth's spin? Or is this incorrect? I could imagine explanations to these questions like to a good approximation we can assume the pendulum to be within an inertial frame, or that the pendulum is "decoupled" from the frame of the centre of the Earth etc, but I would like to hear from you.

[insane_alien]

residual angular momentum from the accretion disk around the early sun. and pendulums are affected by the rotation of earth. you can use them as quite effective clocks if you have a really big one.

[YT2095]

we can assume the pendulum to be within an inertial frame

 

that`s closer to my understanding of it also.

the longer the rope or string, the more massive the weight at the end and better (freer) the pivot at the top, the more "Independant" it will be as there will be little rotational torque added to its movement.

 

it`ll never be 100% perfect and unaffected, but can get noticable results from very simple apparatus

[insane_alien]

http://en.wikipedia.org/wiki/Foucault_pendulum

 

YT, thats about as big as they get. and its coupled to the earths rotation quite nicely.

[YT2095]

yeah, they`ve got a nice one at the Birmingham Science Museum too.

[swansont]

http://en.wikipedia.org/wiki/Foucault_pendulum

 

YT, thats about as big as they get. and its coupled to the earths rotation quite nicely.

 

 

No, actually, it's decoupled from the earth's rotation, which is why it changes its apparent orientation over time.

[abskebabs]

No, actually, it's decoupled from the earth's rotation, which is why it changes its apparent orientation over time.

That makes sense actually, though I cannot help but ask:

Why is it not coupled? Also, why is the Earth spinning in the 1st place .Is it residual angular momentum as insane suggested, and if so should the rate of the Earth's spin actually be slowing down slightly because of resistive/frictional forces it encounters from things in space?(these questions and my own possible ideas for answers I have already alluded to)

[Klaynos]

That makes sense actually, though I cannot help but ask:

Why is it not coupled? Also, why is the Earth spinning in the 1st place .Is it residual angular momentum as insane suggested, and if so should the rate of the Earth's spin actually be slowing down slightly because of resistive/frictional forces it encounters from things in space?(these questions and my own possible ideas for answers I have already alluded to)

 

IIRC it is residual angular momentum, and yep it's slowing down.

[swansont]

That makes sense actually, though I cannot help but ask:

Why is it not coupled? Also, why is the Earth spinning in the 1st place .Is it residual angular momentum as insane suggested, and if so should the rate of the Earth's spin actually be slowing down slightly because of resistive/frictional forces it encounters from things in space?(these questions and my own possible ideas for answers I have already alluded to)

 

The plane of the Foucalt pendulum will stay fixed with respect to an inertial coordinate system. It's probably easiest to envision this if you were on the North or South pole. Swing the pendulum in the direction of some star near the horizon, and it will continue to swing in that direction; the earth will rotate beneath it, which is why the pendulum rotates with respect to the earth — it's actually the earth rotating beneath the pendulum.

 

When you aren't at the pole the roattion takes longer, because your plane is with respect to a projection of the rotation axis, i.e. there's a dot product involved. So, no effect at the equator.

 

The slowing of the rotation of the earth is due to trading angular momentum with the moon, which is receding from us at about 4 cm/year. That causes a deceleration of about 1.5 msec/day/century. This is modified by internal processes that change angular momentum, like mass redistribution or weather patterns.

[YT2095]

 

The slowing of the rotation of the earth is due to trading angular momentum with the moon, which is receding from us at about 4 cm/year. That causes a deceleration of about 1.5 msec/day/century. This is modified by internal processes that change angular momentum, like mass redistribution or weather patterns.

 

just as an aside, does the fact that we have a liquid core as opposed to a solid one have any slowing down effect?

 

the reason I ask is the old trick with 2 eggs, one fresh the other boiled, how do tell which is which without breaking them?

you Spin them, the cooked one will spin very well, the liquid one will not.

 

can that effect affect Earth in anyway?

[swansont]

just as an aside, does the fact that we have a liquid core as opposed to a solid one have any slowing down effect?

 

the reason I ask is the old trick with 2 eggs, one fresh the other boiled, how do tell which is which without breaking them?

you Spin them, the cooked one will spin very well, the liquid one will not.

 

can that effect affect Earth in anyway?

 

Right, one would expect that the rotation of the core will not be fully coupled to the rotation of the mantle and crust. So it has to have an effect, because that changes the effective moment of inertia. But I'm not sure how much difference you have between that and a solid.