Question

Kepler's second law states that the straight line joining the planet to the sun sweeps out equal areas in equal times. This statement is equivalent to saying that

• A. total acceleration is zero
• B. tangential acceleration is zero
• C. longitudinal acceleration is zero
• D. radial acceleration is zero

Answer 1

Answer: (B)

A planet moves around the sun under the effect of a purely radial force. Therefore, areal velocity of the planet must always remains constant. = $\frac{\Delta A}{\Delta t}=\frac{L}{2m}$ = constant vector Therefore, Kepler's 2nd law is the consequence of the principle of conservation of angular momentum (L) $\tau =\frac{dL}{dt}=0$ Now, $\tau =I\alpha$ = $I\alpha =0or\alpha =0$ or $a_T=r\alpha =0$ ie, tangential acceleration is zero.