Kinetics of a Particle

We can also say that"inertial" system is a system in which F = ma is valid.

To understand the concept of inertial system, consider a cart from the roof of which a bob is suspended by a thread.

Whenever the cart accelerates or decelerates, the bob deflects. An observer sitting in the cart will think that bob is accelerating/decelerating without applying any force. Thus, for him the Newton's second law does not hold good. However, the fallacy is that he is applying the Newton's law in non-inertial system. One has to be careful in applying Newton's law to only an inertial frame of reference.