  The compound Atwood's machine (Atm) problem is revisited in order to
introduce young students on advanced concepts in Physics. Atm is an
old-fashioned device. However, it allows us to speak about relativity of
motion, principle of equivalence, inertial and non-inertial frames of
reference, general covariance and invariance under coordinate transformations.
Besides, it also provides experimental support for our theoretical models. We
have started with coordinate transformations and inertial and non inertial
reference frames followed by the principle of equivalence. The composed Atm was
worked out in the following. We calculate the acceleration in the machine
applying Newton's Laws to describe its dynamics in an inertial frame on the
fixed pulley center and in a non-inertial one on the moving pulley center.
Coordinate transformations mapping inertial frame solutions in non-inertial
ones and vice-versa allow students catch on relativity concepts and the role
played by general covariance. A comparison between these solutions showed the
importance of the principle of equivalence in the evaluation of the intensity
of gravity locally. In addition, according to this coordinate transformation
the non-inertial reference frame equations of motion are equivalent to one
single Atm plus one falling mass. Our results have also shown, in agreement
with experimental outcomes, that after movement starts single Atm becomes
lighter than $m_{3}$ mass, consequently $m_{3}$ mass is falling. It is a
non-intuitive experimental result first observed for the 1854 Poggendorff's
fall machine. A measure of single Atm's mass reduction was reported in 2016's
last issue of Physics Education.
