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OK, I'll give it a shot. If one hangs a mass from a spring and then
drives the UPPER end of the spring into oscillation with a sinusoidal
driving force (this is Leigh's example system in an MET post, but with a
sinusoidal driver), this same force is transmitted to the mass by the
driving force (this is Leigh's example system in an MET post, but with a
sinusoidal driver), this same force is transmitted to the mass by the
(massless, frictionless) spring. Now the mass' acceleration will also be
in phase with this force and the mass' displacement is always 180 degrees
away from its acceleration, resonance or no!
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It seems to me that the mass's acceleration will not be in phase with the
force of the spring. At resonance a damping force about the same size as
the force of the spring also acts on the mass. Unless this damping force
is in phase with the force of the spring the mass's acceleration will not
be in phase with the force of the spring.
A colleague sent me some URLs on the mass-tuned damper. I haven't had the
chance to look at them yet so I can't comment on them.