Question

Predict the gravitational force between two 15-kg balls whose centers are 35 m apart.

Answer 1

$F=G\frac{m_1-m_2}{r^2}$

$=\frac{(6.67\times10^{-11}N*m/kg^2)(15kg) }{(0.35m)^2}$

$=1.2\times10^{-7} N$

Because the weight of one ball is mg = 147 N, the gravitational force between the two balls is $1.2\times10^{-7}N$ or $8.4*10^3$ parts per billion of the  weight.