Suppose we have a star and a planet with radius vectors r1 and r2 respectively in a fixed inertial coordinate frame. Relative position of planet from sun is r = r2 - r1(adsbygoogle = window.adsbygoogle || []).push({});

Why is the gravitational pull felt by the planet equals to F = Gm1m2 / r^2 * ( -r/r ) ?

Therefore, F= Gm1m2/r^3

Secondly, we want to find the relative motion of the planet with respect to the star.

Why is it that we have to substract the equation of motion of the star from that of the planet ?

m1 = mass of star

m2 = mass of planet

m2a2 = -Gm1m2r/r^3 ------ 1st eq

m1a1 = Gm1m2/r^3 -------- 2nd eq

Finally, a = G(m1+m2)r/r3

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# Equations of motion

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