An instrument is a concrete or abstract tool intended for a purpose other than mechanical work, in particular a refined one.
Musical instruments are devices designed to produce music, frequently by plucking or striking a string or bell or drum surface, or by forcing air to resonate (see wind instruments).
... the axis pushes against the lower rim of the groove. While the axis is slipping around inside the groove, the friction between the axis and the groove rims will accelerate ...