Line up in a direction parallel to the magnetic field lines<span />
(a) The stone moves by uniform accelerated motion, with constant acceleration

directed downwards, and its initial vertical position at time t=0 is 750 m. So, the vertical position (in meters) at any time t can be written as

(b) The time the stone takes to reach the ground is the time at which the vertical position of the stone becomes zero: y(t)=0. So, we can write

from which we find the time t after which the stone reaches the ground:

(c) The velocity of the stone at time t can be written as

because it is an accelerated motion with initial speed zero. Substituting t=12.37 s, we find the final velocity of the stone:

(d) if the stone has an initial velocity of

, then its law of motion would be

and we can find the time it needs to reach the ground by requiring again y(t)=0:

which has two solutions: one is negative so we neglect it, while the second one is t=11.78 s, so this is the time after which the stone reaches the ground.
Explanation:
increase the distance of cube from black and dull substance
Here we can say that by energy conservation principle
Elastic potential energy of spring will convert into kinetic energy of the block
so here we will have

we also know that
k = 360 N/m
x = 11 cm
m = 1.85 kg
now we will use all in above equation




so it will move with speed 2.17 m/s after separating from spring