  We investigate the initiation and early evolution of 12 solar eruptions,
including six active region hot channel and six quiescent filament eruptions,
which were well observed by the \textsl{Solar Dynamics Observatory}, as well as
by the \textsl{Solar TErrestrial RElations Observatory} for the latter. The
sample includes one failed eruption and 11 coronal mass ejections, with
velocities ranging from 493 to 2140~km~s$^{-1}$. A detailed analysis of the
eruption kinematics yields the following main results. (1) The early evolution
of all events consists of a slow-rise phase followed by a main-acceleration
phase, the height-time profiles of which differ markedly and can be best fit,
respectively, by a linear and an exponential function. This indicates that
different physical processes dominate in these phases, which is at variance
with models that involve a single process. (2) The kinematic evolution of the
eruptions tends to be synchronized with the flare light curve in both phases.
The synchronization is often but not always close. A delayed onset of the
impulsive flare phase is found in the majority of the filament eruptions (5 out
of 6). This delay, and its trend to be larger for slower eruptions, favor ideal
MHD instability models. (3) The average decay index at the onset heights of the
main acceleration is close to the threshold of the torus instability for both
groups of events (although based on a tentative coronal field model for the hot
channels), suggesting that this instability initiates and possibly drives the
main acceleration.
