Kepler's first law for any body

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Discussion Overview

The discussion revolves around the implications of Kepler's first law in the context of objects placed within the solar system. Participants explore how such objects would behave under the influence of the Sun's gravity, including the conditions for stable orbits and the nature of centripetal acceleration.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that an object placed in the solar system will revolve around the Sun if it has sufficient tangential velocity, while others note that insufficient velocity would result in the object falling into the Sun.
  • It is suggested that the type of orbit (closed or open) depends on the object's velocity and distance from the Sun, with specific paths being ellipses, parabolas, or hyperbolas.
  • One participant emphasizes that Kepler's first law is based on observations of known planets and does not apply universally to all objects influenced by the Sun's gravity.
  • There is a discussion about the conditions under which an object might have a stable orbit, highlighting the role of eccentricity and the potential for orbits to intersect the Sun if certain conditions are met.
  • Participants clarify that the mass of the object can be ignored as long as it is significantly smaller than that of the Sun.
  • Some contributions mention that Kepler's laws are observational rules and that Newton's theory provides a more comprehensive understanding, though it is acknowledged that even Newton's theory has limitations.

Areas of Agreement / Disagreement

Participants express a range of views on the applicability of Kepler's laws and the nature of orbits, indicating that multiple competing perspectives remain. There is no consensus on the realism of Kepler's laws or the implications of gravitational interactions among celestial bodies.

Contextual Notes

Participants note that the paths described by Kepler's laws assume massless bodies and do not account for the gravitational interactions between planets, which complicate the orbits. Additionally, the discussion acknowledges that Newton's theory of gravity is not entirely accurate, particularly in light of General Relativity.

Vishaaaal
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I was just wondering, what if an object is placed in our solar system, will it revolve around the sun? If yes, will it have a centripetal acceleration too?
 
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If an object is placed in the solar system with too small a tangential velocity relative to the Sun, then it will fall directly into it. Otherwise it will be put into either a closed or open orbit depending on its velocity.

Vishaaaal said:
If yes, will it have a centripetal acceleration too?

Certainly.
 
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To add to @Drakkith's post, it might orbit or crash into a planet or moon, too. It depends where you put it and how fast it's moving in what direction.
 
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Kepler's first law is based on the observation of the known planets of the time. Objects that were already in closed orbits around the Sun. It does not extend to every object that might be within the Sun's gravitational influence. The ellipse is one solution for the path for such an object, other solutions are a parabola or hyperbola. (and circle, which is really just a special case of ellipse.)
Which of the paths the object follows depends on its velocity and distance from the Sun at at given point. Parabolas or Hyperbolas will be followed by objects that are moving too fast to be in a closed orbit. (the parabola actually is the "dividing line" between a elliptical and hyperbolic path. If you pick any distance from the Sun, there is a velocity at which the object would be traveling in a parabola, Anything slower than this would result in a closed elliptical orbit and anything faster result in a hyperbola.
Even if the velocity is such that you have an ellipse is not a guarantee that you will end up with a stable orbit. This will depend on the "eccentricity" or flatness of the orbit. If the object at our given distance from the Sun is moving too slow our its direction of movement is pointed to much towards the Sun, the orbit will be so eccentric that it orbit will take it into the surface of the Sun. So what we end up with is a range of possible solutions that end up giving us a stable orbit, and anything out side this range does not.

As far as centripetal acceleration goes, all objects within the Sun' gravitational influence are subject to it, as it is the Sun's gravity that produces it.
 
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Drakkith said:
If an object is placed in the solar system with too small a tangential velocity relative to the Sun, then it will fall directly into it. Otherwise it will be put into either a closed or open orbit depending on its velocity.
Thanks mate. Answered my question in the best possible way. So...the orbit does not depend upon its mass right?
 
Vishaaaal said:
Thanks mate. Answered my question in the best possible way. So...the orbit does not depend upon its mass right?

As long as the mass of the object is significantly smaller than that of the Sun, then you can mostly ignore it.
 
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Janus said:
Kepler's first law is based on the observation of the known planets of the time. Objects that were already in closed orbits around the Sun. It does not extend to every object that might be within the Sun's gravitational influence. The ellipse is one solution for the path for such an object, other solutions are a parabola or hyperbola. (and circle, which is really just a special case of ellipse.)
Which of the paths the object follows depends on its velocity and distance from the Sun at at given point. Parabolas or Hyperbolas will be followed by objects that are moving too fast to be in a closed orbit. (the parabola actually is the "dividing line" between a elliptical and hyperbolic path. If you pick any distance from the Sun, there is a velocity at which the object would be traveling in a parabola, Anything slower than this would result in a closed elliptical orbit and anything faster result in a hyperbola.
Even if the velocity is such that you have an ellipse is not a guarantee that you will end up with a stable orbit. This will depend on the "eccentricity" or flatness of the orbit. If the object at our given distance from the Sun is moving too slow our its direction of movement is pointed to much towards the Sun, the orbit will be so eccentric that it orbit will take it into the surface of the Sun. So what we end up with is a range of possible solutions that end up giving us a stable orbit, and anything out side this range does not.

As far as centripetal acceleration goes, all objects within the Sun' gravitational influence are subject to it, as it is the Sun's gravity that produces it.

Can you provide an example to me? Its just that I began taking Physics seriously this year and the laws don't seem too realistic to me.
 
Drakkith said:
As long as the mass of the object is significantly smaller than that of the Sun, then you can mostly ignore it.

All of my doubts are cleared...thanks pal :)
 
Vishaaaal said:
Can you provide an example to me? Its just that I began taking Physics seriously this year and the laws don't seem too realistic to me.
What, Kepler's laws, or physics in general?

Kepler's laws are observational rules of thumb, basically. He says "these things are true of the observed movements of the planets" without providing any explanation. Newton provided the theory a few years later. Ellipses, parabolae and hyperbolae are the solutions to differential equations describing the motion of a free-falling body in a force field with ##1/r^2## behaviour, like gravity.

The reality isn't quite like that. Some solutions are curves that intersect the star. Strictly, the paths are only correct for massless bodies because they assume that the Sun doesn't move under the gravity of the planets (which is very close to true given the mass ratio, but not quite). Also, the planets' gravitational fields affect each other so their orbits aren't as simple as ellipses. Finally, Newton's theory of gravity is slightly wrong, and provides predictions that aren't quite right. That General Relativity accounted for the (tiny) anomalous precession of Mercury's orbit was a major boost to its credibility.
 
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Ibix said:
What, Kepler's laws, or physics in general?

Kepler's laws are observational rules of thumb, basically. He says "these things are true of the observed movements of the planets" without providing any explanation. Newton provided the theory a few years later. Ellipses, parabolae and hyperbolae are the solutions to differential equations describing the motion of a free-falling body in a force field with ##1/r^2## behaviour, like gravity.

The reality isn't quite like that. Some solutions are curves that intersect the star. Strictly, the paths are only correct for massless bodies because they assume that the Sun doesn't move under the gravity of the planets (which is very close to true given the mass ratio, but not quite). Also, the planets' gravitational fields affect each other so their orbits aren't as simple as ellipses. Finally, Newton's theory of gravity is slightly wrong, and provides predictions that aren't quite right. That General Relativity accounted for the (tiny) anomalous precession of Mercury's orbit was a major boost to its credibility.

Kinda got it (y)
 

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