  Evanescent electromagnetic waves possess spin-momentum locking, where the
direction of propagation (momentum) is locked to the inherent polarization of
the wave (transverse spin). We study the optical forces arising from this
universal phenomenon and show that the fundamental origin of recently reported
non-trivial optical chiral forces is spin-momentum locking. For evanescent
waves, we show that the direction of energy flow, direction of decay, and
direction of spin follow a right hand rule for three different cases of total
internal reflection, surface plasmon polaritons, and $HE_{11}$ mode of an
optical fiber. Furthermore, we explain how the recently reported phenomena of
lateral optical force on chiral and achiral particles is caused by the
transverse spin of the evanescent field and the spin-momentum locking
phenomenon. Finally, we propose an experiment to identify the unique lateral
forces arising from the transverse spin in the optical fiber and point to
fundamental differences of the spin density from the well-known orbital angular
momentum of light. Our work presents a unified view on spin-momentum locking
and how it affects optical forces on chiral and achiral particles.
