Air molecules in the atmosphere are attracted vertically downward by gravitational force of the earth just like an apple falling from a tree. Air molecules move randomly due to their thermal velocity and hence the resultant motion of air molecules is not exactly in the vertical downward direction.

But in case of apple, only vertical motion dominates because of being heavier than air molecules. But due to gravity, the density of atmosphere increases near to the earth's surface.

Example of central force Gravitational force, electrostatic force etc.

Example of non-central force Nuclear force, magnetic force acting between two current carrying loops etc.

Areal velocity of a planet revolving around the sun is constant with time. Therefore, graph between areal velocity and time is a straight line (AB) parallel to time axis. (Kepler's second law).

Areal velocity of the earth around the sun is given by

$$\frac{d \mathrm{~A}}{d t}=\frac{\mathrm{L}}{2 m}$$

where, $$\mathbf{L}$$ is the angular momentum and $$m$$ is the mass of the earth.

But angular momentum $$\quad L=r\times p=r\times mv$$

$$\therefore$$ Areal velocity $$\quad \left(\frac{d A}{d t}\right)=\frac{1}{2 m}(\mathrm{r} \times m \mathrm{v})=\frac{1}{2}(\mathrm{r} \times \mathrm{v})$$

Therefore, the direction of areal velocity $$\left(\frac{d \mathrm{~A}}{d t}\right)$$ is in the direction of $$(\mathrm{r} \times \mathrm{v})$$, i.e., perpendicular to the plane containing $$r$$ and $$v$$ and directed as given by right hand rule.