7. Arcs

Most human and animal actions occur along an arched trajectory , and animation should reproduce these movements for greater realism. This can apply to a limb moving by rotating a joint, or a thrown object moving along a parabolic trajectory. The exception is mechanical movement, which typically moves in straight lines.

In computer animation fk mostly give you an automatic arc While in ik you have to manually add keys to make it move in arc this because ik just calculates the shortest distance from point a to b and covers it.