5. As shown in Figure 12, line $l$ is the boundary of the forest. The rabbit and the wolf are located at points $A$ and $B$ on the perpendicular line $AC$ to line $l$ $(AB = BC = a)$.
They run at fixed speeds, with the rabbit's speed being twice that of the wolf: if the wolf arrives at a point earlier than or at the same time as the rabbit, the rabbit will be caught. Suppose the rabbit enters the forest along line segment $AD$. For which points $D$ on line $l$ will the rabbit not be caught by the wolf on line segment $AD$?
5. Draw a Cartesian coordinate system as shown in Figure 14. Suppose the initial positions of the rabbit and the wolf are $A(0,2a)$ and $B(0,a)$, respectively, and their speeds are $2v$ and $v$. If the rabbit runs along a straight line to point $M(x, y)$, then at time $t_{1}=\frac{A M}{2 v}$, it can reach point $M$. The wolf can also reach point $M$ at time $t_{2}=\frac{B M}{v}$. If $t_{1}\left(\frac{2}{3} a\right)^{2}$. (*)
Points whose coordinates satisfy ( ) are called safe. For the rabbit to avoid being caught by the wolf, it is necessary and sufficient that the line segment $A D$ is entirely outside the circle with center $K\left(0, \frac{2}{3} a\right)$ and radius $\frac{2}{3} a$. Draw the tangent line $A D$ from point $A$ to the circle, intersecting line $l$ at $D_{1}$. It is easy to calculate that $\angle O A D_{1}=30^{\circ}$, and $C D_{1}$ $=2 a / \sqrt{3}$, so when $C D>C D_{1}$, $A D$ is a safe line segment.