# Newtons law of universal gravitational question

redruM
http://www.boredofstudies.org/community/attachment.php?s=&postid=266675 [Broken]

when doing this question, isn't the radius of the planet supposed to be added to the altitude of the satellite?
having said that, would the term 'orbit' mean that the satellite is 35 000km away from the centre of mass, ie the centre of the planet and hence the radius is already included?

thanks for any replies

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## Answers and Replies

Fuego
the distance d is the distance between the centres of mass of the two bodies. so yes, the radius of the planet should be added to the altitude of the satellite.

i assume that in that sample question, 35,000km refers to the distance from the planet's centre of mass, otherwise the calculation would be innacurate. however, i would generally understand the orbit altitude to refer to the altitude above the surface of the planet.

as long as you specify what you mean by certain terms, it doesn't really matter (the example you gave doesn't do this). this is physics, not semantics.