<span>the gravational potential energy of anything on the ground is zero. When calculating potential energy you take height in meters and multiply it by the mass of the object in kilograms and the acceleration of gravity to get a new unit called Joules.
Any object at ground level has a potential energy of zero newtons becuase anything multiplied by zero is zero. An object with mass of 54 kg, 4 meters above the ground has a gravitatinal potential energy of 2116.8 Joules.</span>
Answer:
400 N
Explanation:
By the law of friction,

is the maximum frictional force,
is the coefficient of friction and
is the reaction on the refrigerator. On a horizontal surface, the reaction is equal to the weight of the refrigerator.


While not moving, the fricition on the refrigerator is static friction. So, 

This is the maximum frictional force and is more than the applied horizontal force of 400 N. Frictional force cannot be more than the applied force, else it would actually pull the refrigerator backwards (a strange thing, if it were to happen). It is equal to the extent of the applied force because the applied force is not enough to overcome the maximum.
Hence the frictional force is 400 N.
PS: Note that we do not use the coefficient of kinetic friction because applied force could not overcome the static friction.
Work done against gravity to climb upwards is always stored in the form of gravitational potential energy
so we can say

here h = vertical height raised
so here we know that

here we have

now from above equation


so work done will be given by above value
Explanation:
Terminal velocity is given by:

Here, m is the mass of the falling object, g is the gravitational acceleration,
is the drag coefficient,
is the fluid density through which the object is falling, and A is the projected area of the object. in this case the projected area is given by:

Recall that drag coefficient for a horizontal skydiver is equal to 1 and air density is
.

Without drag contribution the motion of the person is an uniformly accelerated motion, thus:

It comes from the sun, and then it is converted to energy/electricity (by solar panels)