A Pseudo force is a fictitious force. A Pseudo force is an apparent force that acts on all masses whose motion is described using a non-inertial frame of reference frame, such as rotating reference frame.
Pseudo force comes in effect when the frame of reference has started acceleration compared to a non-accelerating frame.
Example: Consider a ball hung from the roof of a train by means of an inextensible string. If the train is at rest or is moving with a uniform speed in a straight line the string will be vertical. A passenger will infer that the net force acting on the ball is zero.
