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Keplers Law and radius

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sidzeman:
Hey so for kepler law of periods (rcubed on T^2 and so), what exactly is r?

 I did some questions and it seems to be (in the example of Earth and the Sun) the distance from the Earth to the Sun + the radius of the Sun - why isn't the radius of the Earth included? Thanks!

banway:
I believe the Radius of the earth should be included in that.

Or it may be that the Radius of the earth is taken into account when they give you the distance.

Not 100% sure, sorry man

jamonwindeyer:

--- Quote from: sidzeman on August 08, 2017, 08:49:15 pm ---Hey so for kepler law of periods (rcubed on T^2 and so), what exactly is r?

 I did some questions and it seems to be (in the example of Earth and the Sun) the distance from the Earth to the Sun + the radius of the Sun - why isn't the radius of the Earth included? Thanks!

--- End quote ---

For Kepler's \(r\) is always the distance between the centres of the two objects, so you would definitely need to include Earth's radius, at least in theory. However, when compared to the distance between the Earth and Sun, Earth's radius is minuscule - Adding it or not won't change your answer in any significant way. It is general practice to, when the object orbiting is far smaller than the object it orbits (in most cases), ignore the radius of the smaller orbiting object. Just take the distance they give you and add it to the radius of what you are orbiting, for ease :)

sidzeman:
ohhhh okay thanks guys!

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