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 .