Another striking example of the magnitude of the atmospheric
pressure is offered by the so-called Magdeburg
disks. To semi-spheres with a smooth mating surface between them
are pressed together (without any mechanical attachment) and then
the volume between them is evacuated. As a result of the
atmospheric pressure on the outer surface of the sphere
the two parts of it are pressed so hard together that it is
not easy to pull them apart. Specifically the force acting between
the two parts of the sphere is
Where the factor of two comes from adding the opposed forces
acting on each of the two semi-spheres. Putting in numbers
for our apparatus we get:
Which is the force of gravity acting on a mass of 57 kg. It takes a strong person to pull the semi-spheres apart and all you are doing
is counteracting the atmospheric pressure!