please tell me when to consider centripetel and centrifugal forces .explain with example.

consider the example of a pendulum bob if it swings in such a way that if you trace its line of motion it will be a circular one.consider the position of the bob when it is swinginging towards left.   

join the bob to the centre of its circular path  and join thefixed part of the bob with the centere of its circular path again.it willm come out tobe a right triangle.consider the vertical angle   .

make a free body diagram of the bob-mg downwards and T along the string .resolute  T _tcosupwards

and tsintowards the centre of the cicular path.in this case this is the centripetal force. centripetal force is any force that makes the object move towards the centre of the circle.9THIS IS THE EXPLANATION FROM INERTIAL FRAME.AS THERE IS NO FORCE TO RESIST THE MOTION OF THE BOB TOWARDS THE CENTRE ,THERE IS NO CENTRIFUGAL FORCE.

 NON-INERTIAL FRAME.

if you make an observer sit on the moving bob(non-inertial observer)he will assume that the bob is at rest inspite of thecentripetal force.aforce must be pulling it outwards.thispseudo force is the centrifugal force in this case.centrifugal force is the force that resists the inward cicular motion of the bob.