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Here, we have the weight force W
decomposed in its components
Ws
in the direction of the string
and
Wm
perpendicular to the string which is also in the direction
of the motion of the mass.
Since the string tension S
is a reaction force, it can only compensate the component of the
weight force which is
in the direction of the string.
Consequently,
S = - Ws
The component of the weight
Wm
which is in the direction of the motion of the mass,
is the restoring force which
gives rise to the acelleration of the mass.
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