The planet mercury is revolving in an elliptical orbit around the sun as shown in the figure. The speed of mercury will be greater at the point:

CONCEPT:

Johannes Kepler proposed laws of planetary motion.

• Kepler’s First law: Every planet revolves around the Sun in an elliptical orbit and Sun is situated at one of its two foci. It is also termed as ‘the Law of Orbits’.
• Conservation of Angular Momentum: The velocity and distance from the Sun both change as the planet moves in an elliptical orbit, but the product of the velocity times the distance stays constant.

L = m v r,

Where m is the mass of the planet, v is the planet's orbital velocity and r is the distance can be taken as the semi-major axis of the orbit (the distance between sun and planet).

EXPLANATION:

As from the law of conservation of angular momentum; we can say that the speed of the mercury will be the maximum when its distance from the sun is the minimum;

∴ m v r = constant

So, the speed of planet mercury at a maximum at point ‘A’ because the distance between the sun and the mercury is the lowest at ‘AS’ in the figure.