ICSE Class 10 Answered

Its because newton laws of motion is only valid in inertial frame of reference. 

Like in well of depth the car is moving of the surface of well so the centripetal force is acting on the car . But there is person sitting in the car,  what about him, his i not running directly.  Since he is sitting in car then there must be some force acting on him , but we can't observe the forces directly on him because he is sitting in an non inertial fram (accelerated frame). So when we observe him to get him in the inertial frame we apply a pseudo force on him this pseudo force or imaginary force is called centrifugal force and its direction would be opposite to direction of acceleration of frame. Like centripetal force acting on the car is towards the centre of the well but pseudo force will act away from the centre.