Explanation:
a) The height of the ball h with respect to the reference line is

so its initial gravitational potential energy
is



b) To find the speed of the ball at the reference point, let's use the conservation law of energy:

We know that the initial kinetic energy
as well as its final gravitational potential energy
are zero so we can write the conservation law as

Note that the mass gets cancelled out and then we solve for the velocity v as



C.
Thanks me later, that's my answer.
The acceleration of gravity is inversely proportional to
the square of the distance from Earth's center.
The acceleration of gravity is 9.8 m/s² on the Earth's surface ...
6380 km from the center.
If the acceleration of gravity at 'h' is 4.9 m/s² ... 1/2 of what it is
on the surface, then the distance from the center is
(6380 x √2) = 9,023 km (rounded) ,
and 'h' is the distance above the surface
= (9,023 - 6,380) = 2,643 km (rounded) .