What will happen if two planets faceing each other all the time (just like our moon) revolve each other in an eliptical orbit such that; at regular intervals they come very near to each other creating huge land tides.

As they passby close to each other, both of them get streched and chruned with Tidal forces and large amounts of heat is generated within them (just like in case of Jupiter moons) and they return back to their spherical shape as they move away in the orbits (again generating heat).

Since there is noway to decay of their orbits, from what the heat energy getting converted from ?

Any problems with this system ? a PMM that actually generates Energy.



*Conditions Applied.


*** There is nothing else near them to affect them in any way.

*** Just like moon they always face eachother at the same point.

*** They passby eachother without any kind of friction.

*** Hence there is nothing to decay their orbits.

Your premise is impossible. In eliptical orbit, the angular speed of the line connecting the centers of the two bodies is _not_ constant.

Aside from the tidelocked problem, which doesn't really have that much bearing on the question, this won't work. Due to gravity not being an instantaneous phenomenon, the tidal bulge on both planets won't be perfectly aligned with the line through the two planets, but slightly behind this line. Thus, there is a slightly greater gravitational attraction from behind the planet, marginally slowing the speed of orbit.

Energy is not created.

Your premise is impossible. In eliptical orbit, the angular speed of the line connecting the centers of the two bodies is _not_ constant.

why not ?

When the planets come nearest to eachother they are always facing at the same point, just like moon is facing us.

Obviously their total time of revolution and rotation are the same, regardless the shape of their orbits .

.... Thus, there is a slightly greater gravitational attraction from behind the planet, marginally slowing the speed of orbit.

Energy is not created. I admit I am dumb and harass lot of people here, but

I didnt get your point why will the orbits decay in this case.

An eliptical orbit is like a figure skater that alternately pulls in and then extends his/her arms during a spin. No planet can lock to the angular orbital position of its companion because the angular speed of that companion is always changing. Now you have a situation where the orbital bulge is not at all times locked the the orbit and energy transfers can occur. Generally, the orbital bulge will oscillate (called nutation, iirc) and if one or both of the bodies are viscous (friction) then there will be net energy transfer.

http://img201.imageshack.us/img201/3680/planetspj1.th.gif (http://img201.imageshack.us/my.php?image=planetspj1.gif)
Apologies for the slightly sketchy diagram. :)
I'll have another go - the tidal bulge of each planet is not aligned with the line through their centres, due to gravity travelling at the speed of light. This means that the attraction between the two planets is also not along this line, so the orbit slows slightly on each rotation, so the planets lose kinetic energy, which is transferred to the heat created by tidal friction.

why not ?

When the planets come nearest to eachother they are always facing at the same point, just like moon is facing us.

Obviously their total time of revolution and rotation are the same, regardless the shape of their orbits .

But that does't mean that they will keep the same fases to each other at all times. The orbital velocity of an eliptical orbit changes as from closest approach to furthest distance. It is fastest at the point of nearest approach.

So starting at furthest distance, with sides A facing each other, the angular velocity of the orbit will be less than that of the palnet's rotations, and they will spin faster than then orbit. This will continue until, as the planets get closer to each other in their orbits, they reach a point where orbital revolution and planet rotation equal each other. By this time sides A have rotated quite a bit ahead of the orbit and no longer face each other. As the orbits pull the planets closer, the orbital speed begins to exheed rotation speed. Sides A (with respect to the orbit) begin to drift back to facing each other, until at closest approach, they once again face each other. Then on the outward bound half of the orbit, the sides drift off again and then drift back to face each other at furthest distance.

The upshot is that during the half of the orbit when the planets are closest to each other, the planets orbit faster than they rotate and on the other half they rotate faster than they orbit. This means that the tidal bulges alternately lag behind or pull ahead of the line joining the two planets. This tidal interaction causes the planet's rotation to slow, and the average distance between them to increase during part of the orbit, and for the rotations to speed up and the average distance to decrease for the other half.
(the actual mechanism causes the perapis (closest distance) to increase, and the apapis(furthest distance) to decrease, which has a circulizing effect on the orbit over time.) But, since the distance decrease, rotation increase part happens during closest approach, and the tidal interaction is strongest then (tidal forces fall off by the cube of the distance), this period has the overriding effect. The rotations will speed up over time and the average orbital distance will decrease in size, the resulting smaller orbits have less energy than the original orbits, and it is from this loss of orbital energy that the energy for the heating come from.

No energy is generated, it is just transformed from one type to another.

Your assertion that there is nothing to change the orbits is simply wrong.

so u mean that all elliptical orbits decay to form circular ones ?

so u mean that all elliptical orbits decay to form circular ones ?

In a simple two-body under no outside influence or in instances where the outside influences are very minimal, yes.
Of course, in the real universe, there are the tugs and pulls of other bodies to consider, and these can tend to tug the orbit to be elipitical (at the expense of their own orbital energy). There will always be a complex process of pertubations that make orbits more circular or more eliptical, and transfer energy from one orbit to another, but they all involve a transfer of energy.

A good example of tidal interaction tending to circularize orbits can be seen in the Jovian system. Jupiter's gravity has such a strong influence that the innermost moons have orbits of almost exactly 0 eccentricity. (They are not exactly 0 because the other moons do have some influence on them. In fact, it is Europa's gravitational influence on IO that causes it moving slighty in and out in its orbit and to undergo libration. It is this variation that causes the tidal heating of IO. Of course, this is done at the expense of Europa's orbital energy.)

An orbit which consists of only two mutually gravitating bodies cannot be (perfectly) circular unless one of the bodies has infinite mass relative to the other. Otherwise the two bodies must orbit foci which are NOT identical.

The definition of a circle is: an ellipse in which the two foci ARE identical.

The definition of an ellipse is: an ellipse in which the two foci are NOT identical.