Weightlessness It is a situation in which the effective weight of the body becomes zero, Weightlessness is achieved( i) during freely falling under gravity( ii) inside a space craft or satellite( iii) at the centre of the earth( iv) when a body is lying in a freely falling lift.
Kepler’ s Laws of Planetary Motion
( i) Law of orbit Every planet revolve around the sun in elliptical orbit and sun is at its one focus.
( ii) Law of area The radius vector drawn from the sun to a planet sweeps out equal areas in equal intervals of time, i. e., the areal velocity of the planet around the sun is constant.
Areal velocity of a planet dA / dt = L / 2m = constant
where L = angular momentum and m = mass of the planet.