Explanation

(b) Earth and all other planets move around the sun under the effect of gravitational force. This force always acts along the line joining the centre of the planet and the sun and is directed towards the sun. In other words, a planet moves around the sun under the effect of a purely radial force. Therefore, areal velocity of the planet must always remain constant.
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{\mathbf{\mathrm{\Delta}}\mathbf{A}}{\mathrm{\Delta}t} = \frac{\mathbf{L}}{2m} = a\ constant\ vector$
Therefore, Kepler’s 2nd law is the consequence of the principle of conservation of angular momentum $(L)$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \tau = 0$
$Now,\ \ \ \ \ \ \ \ \tau = I\alpha\ $
$\therefore\ \ \ I\alpha = 0\ \ or\ \ \alpha = 0$
$or\ \ \ \ \ \ \alpha_{T} = r\alpha = 0$
$ie,$ tangential acceleration is zero.