In order to understand the possible cause of a 'right side stomach pain', it may be helpful to know the organs and structures that exist in this region of the body. Diseases, infections, or irritation of any of these organs will lead to a right side abdominal pain.
At that one instant, its velocity is zero, even though its acceleration is still 9.8 m/secÂ². But velocity cannot REMAIN at zero for any length of time if the acceleration is nonzero.
The acceleration of the object at an instant t i.e. instantaneous acceleration is nearly equal to average acceleration in small time interval $\Delta t$, because in this time interval the acceleration of the object can be considered to be uniform, even though the object has non-uniform acceleration.