Grain of sand attracting the sun? My friend keeps telling me that according to physics...
"The sun attracts a grain of sand on the earth with the same force that the grain of sand attracts the sun"
or
"A grain of sand on the earth attracts the sun"
Is that true? Does in theory a grain of sand really attract the sun?
 A: Yes, it is.
Applying basic principles articulated by Newton the 17th Century, every pair of bodies with mass attract each other in the form of gravity.  That force is proportional to the product of their masses and inversely proportional to the distance between them.
So every grain of sand does exert a gravitational force on the sun.  But using another principle from Newton, Force = (mass)x(acceleration).  So while there is a force on the sun, the sun's mass cancels out of the calculation of acceleration. As the grain of sand's mass is so tiny, the acceleration the sun experiences as a consequence of the force exerted by the grain of sand is practically non-existent.
Formalizing these statements, let M be the mass of the sun, m the mass of the grain of sand.  The magnitude of the gravitational force exerted by the grain of sand on the sun is
$$F = \frac{GMm}{r^2} $$ where $G = 6.63 \times 10^{-11}$ and $r = 1.5 \times 10^{10} \ m$ 
and thus the acceleration the sun experiences towards the earth as a consequence of the grain of sand is
$$ a = \frac{Gm}{r^2} \approx 3 \times 10^{-34} \ m/s^2 $$
where we approximated $m = 1 \ gram$.  Hence the acceleration is not zero, but it is functionally nothing.
