  Context: Transverse loop oscillations and loop contractions are commonly
associated with solar flares, but the two types of motion have traditionally
been regarded as separate phenomena.
  Aims: We present an observation of coronal loops contracting and oscillating
following onset of a flare. We aim to explain why both behaviours are seen
together and why only some of the loops oscillate.
  Methods: A time sequence of SDO/AIA 171 \r{A} images is analysed to identify
positions of coronal loops following the onset of M6.4 flare
SOL2012-03-09T03:53. We focus on five loops in particular, all of which
contract during the flare, with three of them oscillating as well. A simple
model is then developed for contraction and oscillation of a coronal loop.
  Results: We propose that coronal loop contractions and oscillations can occur
in a single response to removal of magnetic energy from the corona. Our model
reproduces the various types of loop motion observed and explains why the
highest loops oscillate during their contraction while no oscillation is
detected for the shortest contracting loops. The proposed framework suggests
that loop motions can be used as a diagnostic for the removal of coronal
magnetic energy by flares, while rapid decrease of coronal magnetic energy is a
newly-identified excitation mechanism for transverse loop oscillations.
