The potential energy of a 30N ball on the ground will be zero. With respect to height, h. Potential energy will be calculated like this. P=mgh. So if its on the ground relatively speaking the h=0. Thus inputting into the above formula. P=0.
The transition from gas to liquid is called condensation. An example would be water droplets forming on an ice cold glass placed in room temperature.
While plane is moving under tailwind condition it took time "t"
so here we will have

here net speed of the plane will be given as


similarly when it moves under the condition of headwind its net speed is given as

now time taken to cover the distance is 2 hours more

now solving two equations

solving above for v_w we got

There is no SI "base unit" of density.
(Any unit of mass) divided by (any unit of volume) is
a valid unit of density.
The units of density that are seen most often are
(gram per cm³) and (kgm per meter³) .