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{GMr}{R^3} & \mbox{if $ r < R $}\\

\frac{GM}{r^2} & \mbox{if $ r \ge 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 $?