The gravitational force exerted by the planet Earth on a unit mass at a distance $r$ from the center of the planet is
$$F(r)=\left\{\begin{array}{ll}{\frac{G M r}{R^{3}}} & {\text { if } r < R} \\ {\frac{G M}{r^{2}}} & {\text { if } r \geqslant R}\end{array}\right.$$
where $M$ is the mass of Earth, $R$ is its radius, and $G$ is the
gravitational constant. Is $F$ a continuous function of $r ?$