## formula for gravity/circular motion in terms of V? (i think...)

If F=mg, and Fc=Fg, Fc=mv2/r (thats v squared; my apologies, i have no idea how to use the superscript button), and Fg=Gm(1)m(2)/r2 (squared), where m(2) is the mass of, say, the earth, and m(1) is the mass of the object orbiting the earth. By substituting these equations together, we get

mv2/r=Gm(1)m(2)/r2,

and we can cancel out the r on the left, and the m (which is mass). but I have a problem; which mass is canceled out on the right side? is it the mass of the object or mass of the earth?

thanks
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 Mentor Blog Entries: 1 Which do you think? Which mass is executing circular motion as it orbits the other? Which mass are you finding the centripetal force on?
 I think i am supposed to be finding it with respect to the earth, so I believe then the mass of the earth is what is left

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## formula for gravity/circular motion in terms of V? (i think...)

 Quote by tman12345 I think i am supposed to be finding it with respect to the earth, so I believe then the mass of the earth is what is left
Right.

Force on orbiting object due to gravity: $$F = GM_{earth}M_{object}/R^2$$

Applying Newton's 2nd law to orbiting object: $$F = M_{object}V_{object}^2/R$$

Combined:

$$M_{object}V_{object}^2/R = GM_{earth}M_{object}/R^2$$

$$V_{object}^2/R = GM_{earth}/R^2$$
 Cancel out mass of object in revolution,thats the mass acted upon by the centripetal force.
 thanks a lot! that really helped. next time ill try to not leave my physics book at school over the weekend

 Tags force of gravity, gravity