Final answer:
The central phrase for the web is likely 'acceleration towards the center' or 'centripetal acceleration' as it is a key concept in circular motion physics, where an object's acceleration is directed towards the center of its path.
Step-by-step explanation:
The phrase that belongs in the center of this web is likely "acceleration towards the center" or "centripetal acceleration"(ac). This concept is crucial in the study of circular motion, where the acceleration is always directed toward the center of the circle, indicating that the object is constantly changing direction as it moves along the curved path. The phrase 'whichever expression is more convenient' suggests that multiple terms or expressions could correctly represent this concept, providing flexibility in how the idea is communicated.