Answer: (a) The minimum velocity needed for a body to escape the earth’s gravitational field; (b) motion of a body under the earth’s gravity alone.
(a) Escape velocity is the minimum velocity with which a body must be projected from the surface of a planet so that it never returns to the planet. For the earth it is about 11.2 km/s and is given by \(v_{esc} = \sqrt{\dfrac{2GM}{R}}\).
(b) When a body moves only under the force of gravity, with no other force such as air resistance acting on it, its motion is called free fall. Near the earth’s surface it has a constant acceleration g, directed vertically downwards.