The planetary temperature energy balance is obtained by radiating back the absorbed radiation energy from outer-space, by the planet and thus acquiring thermal equilibrium.
What is the process of attaining thermal equilibrium by Earth?
The Stefan-Boltzmann law states that the more the temperature a planet has, the more it will radiate out to reach thermal equilibrium.
We know that outer space contains large masses of radiative energy freely distributed in its vast expanse. A small fraction of this energy is absorbed by the Earth through the atmosphere, surface land, clouds etc.
Now, radiative balance is achieved when a planet's surface continuously warms up until it reaches its peak at which point the same amount of absorbed energy can then be radiated back to space. The relative amount of energy radiated back by a planet is dependent upon the size of the planet.
A colder planet relatively absorbs lower amount of radiation energy from space. In some time, as the planet heats up enough, the energy is radiated back to the space attaining thermal equilibrium.
Learn more about Stefan-Boltzmann law here:
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A=F/m
a=(3000000)/(20000)
a=15 m/s^2
Answer: An 8 kg book at a height of 3 m has the most gravitational potential energy.
Explanation:
Gravitational potential energy is the product of mass of object, height of object and gravitational field.
So, formula to calculate gravitational potential energy is as follows.
U = mgh
where,
m = mass of object
g = gravitational field = 
h = height of object
(A) m = 5 kg and h = 2m
Therefore, its gravitational potential energy is calculated as follows.

(B) m = 8 kg and h = 2 m
Therefore, its gravitational potential energy is calculated as follows.

(C) m = 8 kg and h = 3 m
Therefore, its gravitational potential energy is calculated as follows.

(D) m = 5 kg and h = 3 m
Therefore, its gravitational potential energy is calculated as follows.

Thus, we can conclude that an 8 kg book at a height of 3 m has the most gravitational potential energy.
Speed = (frequency)x(wavelength).
Frequency = 6 Hz
Wavelength = 9mm = 0.009m.
Speed = (6 Hz) x (0.009m) = 0.054 m/s or 54 mm/sec.
Pressure is the force per unit area. This means that the pressure a solid object exerts on another solid surface is its weight in newton’s divided by its area in square metres