perpetual motion

[fraserjim]

If a hole were somehow drilled right through the centre of the earth, and an object were dropped into this hole, would the object first accelerate until it reached the centre and then begin to decelerate until the pull from the centre captured it again just after it had emerged from the other end of the hole? Would this oscillating cycle then repeat ad-nauseum, and if so does this constitute a kind of perpetual motion?

[del_1690]

yes, yes and yes.

[IndieSinger]

I have heard a theory that it would just get stuck in the dead centre though.

[sft42]

Yes, yes and no.....The effect of air resistance would gradually slow the reciprocal rate untill the initial kinetic energy had been disippated, only in a perfect vacuum would this have any remote chance of working. I suppose working out the rate of acceleration as being 9.81 metres per second per second and the deccelleration would be the same if you knew the mass of the object and could quantify the loss due to resistance you could in effect work out how long it would oscillate for.

[Caladon]

What can i add to ST4's reply?! Aprt from even in a perfect vacuum the object would eventually slow down due to the strength of the gravitational pull and it would be less and less, though over a longer period of time due to there being a vacuum.

However, and bear with me here...If we were able to drill a hole, say a metre in diametre though the earths core, say from pole to pole...it would mean that the earth would no longer be molten (no more nuclear isotopes or reactions to burn). So if it were dead on the inside, this would affect its electromagnetic force (it would be either weaker or not at all) causing the atmosphere to drift into space causing the vacuum needed

[incitatus]

the atmosphere isnt held in by electromagnetic forces cala, its held in by gravity. You would lose some due to the solar wind, but you would still have some left

[darth vader]

i'm sure we've had this before...... sideways momentum caused by the constant rotation of the planet, will cause the object to start bouncing around the sides of the hole as it falls so you can't use the old pendulum algorithm if you're about to start working it out sft? (errr. 1 over pi times root lg or something like that?.....)

[Phantaxus]

The centre of the earth is the only place on the planet with zero gravity. So you should stop.

[Phantaxus]

I had been wondering about this myself. There was a documentary on discovery channel a few weeks ago about a theoretical journey to the centre of the Earth. I might still be in their current play list.

[Caladon]

sorry inc, but magnetism is what keeps the atmosphere in place. take mars. it has a strong gravity, yet it has no atmosphere, but once had one. try looking on the nasa website

[Hippy]

The atmosphere is kept in place by gravity. To be sure, the planet's magnetic field interacts with parts of the atmosphere to create climate, but it is not the force that keeps it there. See http://coolcosmos.ipac.caltech.edu/cosmic_kids/Ask
Kids/earthatmos2.shtml for what astonomers say. I note that the Nasa Web site says, "Life on Earth is supported by the atmosphere, solar energy, and our planet's magnetic fields." I suppose that could be mis-read to seem that magnetism had something to do with capturing an atmosphere. See http://www.nasa.gov/audience/forstudents/9-12/feat
ures/912_liftoff_atm.html for more.

[Caladon]

Interesting. I did a search on ask.com and had half and half answers. Will email patrick moore (no seriously i will) and ask, because it was he who told me a few years ago that its magnetism which determines whether or not the atmosphere disappears or not.

watch this space

[sft42]

Ok then....assuming we could overcome the problems of having any object hitting the sides of the hole due to rotational velocity, and assuming that we could create a prefect vacuum but still retain the magnetic field and ergo the gravitational pull of the earth.....would this then be possible...after all it is a theoretical question to begin with so??? I still don't think so as even though the conservation of energy must be maintaned perpetual motion must be nigh on impossible to achieve.

[incitatus]

first the magnetism-this allows us to keep a thicker atmosphere than we might otherwise, by sheilding us from the solar wind. The earth would, however, have an atmosphere without it, it would just be a little thinner. Mars has an atmosphere. so does the moon. they are thinner than ours because the gravity is less. Mars atmosphere is largely co2. SFT- it would still run down but for complicated reasons. these include: thermal crossover- the repeated gravitational variations heat the object, dissipating its energy; tides- different ends of the object experience different g forces, inducing spin; relativistic effects....etc. etc. all small, but they count.

[darth vader]

ok sft42 to answer the question after eliminating all external possibilities (momentum, resistance, tidal forces cross-over whatsits, heat, magma getting in the way etc) my answer is no, perpetual motion is not possible. The creation of motion in this case is through gravitational pull and so without external forces the object would "seem" to oscillate perpetually. But in reality the gravitational pull is not uniform and is slowing down as the planet's rotation slows down (all be it at an infinitessimally slow rate) and therefore the oscillations will slowly decrease in magnitude until the object suspends at the centre of the planet (in a few billion years time)

[incitatus]

Deity on a bike- Darth, gravitation is not linked to motion. so a planets gravitational attraction is not linked to rotational motion, and will not decrease as it decreases. the crossover whatsits are the answer because these are the mechanisms by which entropy is entering this situation, and the second ,law of thermodynamics allways wins.

[Disco Fever]

All this stuff is obviously all going above my head but isnt it that hot in the centre of the earth that anything that gets down there would melt anyway, sorry had to say it.