Let's consider -- what ARE the forces acting on the pendulum?

1
it's the pendulum'sinerta
that makes it swing straight out and the force of
gravity
that pulls it straight back.
(or rather "down"--it's the force of the wire that makes it go in an arc rather than straight down.)
2and
air resistance
makes it swing in shorter arcs --but just as straight arcs
(in the demonstration, the electromagnet booster merely counteracts this air resistance)
3also minor disturbances such as
air currents
may throw its path an inch or two off center.
4however, since it is tied to the building, the pendulum will
travel laterally
as the building moves laterally--
but
because of the way it is suspended
twist
around if the building twists around.
so

if the pendulum seems to rotate with respect to the floor--and we know there is no force available to make the pendulum rotate --THEN-- it must be the floor that is rotating--and--if the floor is attached to the earth--THEN

earth rotate

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