- #1
Bjarne
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How is the kinetic energy of a rotation planet (e.g; the earth) calculated?
How does gravitational anomalies affect a planet's rotation?
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Discussion Overview
The discussion revolves around the effects of gravitational anomalies on a planet's rotation, exploring concepts related to kinetic energy, angular momentum, and the implications of mass distribution changes on rotational dynamics. The scope includes theoretical considerations and conceptual clarifications regarding planetary motion and gravitational influences.
Discussion Character
- Exploratory
- Technical explanation
- Conceptual clarification
- Debate/contested
Main Points Raised
- Some participants inquire about the calculation of kinetic energy for rotating planets, mentioning the formula that includes both translational and rotational components.
- There is a suggestion that angular momentum could affect the rotation of planets, with questions raised about the existence of rotating moons.
- One participant asserts that the angular momentum of a planet about its star is constant due to the absence of torque from gravitational forces.
- Another participant proposes that gravitational anomalies, such as density variations or tidal waves, could create an imbalance in mass distribution, potentially affecting a planet's rotation and angular momentum.
- A later reply discusses how tectonic movements could alter a planet's moment of inertia, suggesting that this would impact the period of rotation while maintaining constant angular momentum.
Areas of Agreement / Disagreement
Participants express various viewpoints on the relationship between gravitational anomalies and rotation, with no consensus reached on the implications or mechanisms involved. The discussion remains unresolved regarding the effects of these anomalies on planetary motion.
Contextual Notes
Limitations include assumptions about the constancy of angular momentum and the specific conditions under which gravitational anomalies might influence rotation. The discussion does not resolve the complexities of how these factors interact.
- #2
tiny-tim
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Hi Bjarne! 
But why would anyone want to calculate it?
KE of a rotating body = 1/2 mv2 + 1/2 Iω2
where I is the moment of inertia of the planet about its polar axis, and ω is the angular velocity
(don't forget to include both the spin and orbital angular velocity, eg the Earth's is 2π x 366.25 radians per year, not 2π x 365.25)
Bjarne said:
But why would anyone want to calculate it?
It's not as if anyone would consider trying to change its orbit, is it? 
KE of a rotating body = 1/2 mv2 + 1/2 Iω2
where I is the moment of inertia of the planet about its polar axis, and ω is the angular velocity
(don't forget to include both the spin and orbital angular velocity, eg the Earth's is 2π x 366.25 radians per year, not 2π x 365.25)
- #3
Bjarne
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Thank's tiny-tim
Is it possible that angular momentum affect the rotation of planets?
Does it exist rotating Moons?
Is it possible that angular momentum affect the rotation of planets?
Does it exist rotating Moons?
- #4
tiny-tim
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the angular momentum of a planet about its star (or a moon about its planet) is constant …
this is because the rotational version of Newton's second law says …
and since the torque (moment) of the gravitational force is obviously zero (about the star), the angular momentum cannot change
this is because the rotational version of Newton's second law says …
torque = rate of change of angular momentum
and since the torque (moment) of the gravitational force is obviously zero (about the star), the angular momentum cannot change
- #5
Bjarne
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What I had in mind, what about if a planet or a star had a heavy gravitational anomaly at the surface, a tidal wave, a mountain a crust density anomaly etc.. This would make the planet heavier in the one "end" as in the other "end" .tiny-tim said:the angular momentum of a planet about its star (or a moon about its planet) is constant …
this is because the rotational version of Newton's second law says …
torque = rate of change of angular momentum
and since the torque (moment) of the gravitational force is obviously zero (about the star), the angular momentum cannot change
Would that not cause larger angular momentum in the "heavy end" of the planet, and cause a planet to rotate.
For example the Moon, the more massive side of the Moon point in the direction of the Earth.
Imaging the massive part was pointing opposite and the orbit speed of the Moon would be 30 times faster, - would that not cause the Moon to begin to rotate ?
Last edited:
- #6
tiny-tim
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Bjarne said:
a tectonic plate movement (an earthquake) would alter the moment of inertia (the distribution of mass) …
since the angular momentum must remain constant, that would alter the period of rotation (the length of the day)
(of course, this has nothing to do with energy)
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