A block of mass m is sliding down a smooth vertical circular track....

When a block slides down a smooth vertical circular track, its speed $v$ continuously changes due to the tangential component of gravity.

The tangential acceleration is $a_t = g \sin \theta$, where $\theta$ is the angle made by the radius vector with the vertical.

The radial acceleration is $a_r = \dfrac{v^2}{R}$, directed towards the center of the circular track.

Since the block has both radial and tangential accelerations, the net acceleration of the block is non-zero at all times.

Consequently, the net force acting on the block is non-zero in any direction (horizontal, vertical, or radial).

Therefore, the block is not in equilibrium in any direction.