Answer
$\lim\limits_{r \to 0^{+}}F(r)$ = +$\infty$
Work Step by Step
According to Newton’s Law of Universal Gravitation:
F(r) = G$\frac{Mm}{r²}$, where r is the distance between the centers of the masses. G, M and m are constants. So as they get closer and closer together, $r\to0$:
$\lim\limits_{r \to 0^{+}}F(r)$ = $\lim\limits_{r \to 0^{+}}G\frac{Mm}{r²}$ = +$\infty$. That means the force increases becoming very large. Of course, r is the distance between the centers of the masses, so the distance can't be 0. In a distance r $\gt$ 0 the masses will be touching each other.
