Q.What are inertial frames?

Centrifugal force is a pseudo-force that appears only when describing motion from a rotating (non-inertial) reference frame; it has no existence for an observer in an inertial frame.

In an inertial frame (one that is not accelerating), Newton's laws hold directly and only real, physical forces (like gravity, tension, normal force, friction) are needed to explain motion — a centripetal force is what causes circular motion, but there is no outward 'centrifugal force' acting on the body.

However, an observer who is themselves rotating (a non-inertial, specifically a rotating, frame) sees objects behave as if an additional outward force is pushing them away from the centre. This apparent force, needed only to make Newton's second law appear to hold in that rotating frame, is called the centrifugal force. It is a pseudo-force — it does not arise from any physical interaction, but purely from the acceleration of the observer's own rotating frame of reference.

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Centrifugal force is a pseudo-force introduced only in a rotating (non-inertial) reference frame to account for the effects of the frame's own acceleration.

In an inertial frame (at rest or moving with constant velocity), the real force acting on a body moving in a circle is the centripetal force, directed towards the centre, which is what actually causes the circular motion -- there is no outward 'centrifugal force' truly acting on the body in this frame.

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