What must always be present to cause a centripetal acceleration, according to Newton's second law of motion?

1. Newton's second law states that a net external force is required to produce any acceleration. 2. Centripetal acceleration is still an acceleration, so it too requires a net external force to exist. 3. A constant angular velocity or a fixed radius describes the motion's geometry, not the cause of the acceleration, and a frictionless surface is not required at all, since many other forces can supply the needed force. 4. Only a net external force satisfies Newton's second law as the cause of the centripetal acceleration. _Source: OpenStax College Physics (CC BY 4.0), Ch 6 "Uniform Circular Motion and Gravitation", section 6.2 Centripetal Acceleration_