Q.Define centripetal force.

Centripetal force is not a distinct fundamental force in its own right -- it is simply the NAME given to the direction (towards the centre, centre-seeking) of whatever real force, or combination of real forces, is actually responsible for a body's circular motion: it might be string tension, gravity, friction, or a normal reaction, depending on the situation. The magnitude of this resultant centripetal force is $F_{cp}=mr\omega^2=\frac{mv^2}{r}$. …