i know these topics are discussed many times and i have read many of them but still have a doubt. Suppose a car is accelerating and there is an object and a man in the car. Object is ofcourse at rest with respect to man. The man does not know if the car is accelerating or not - he just sees the object at rest. So for him the car is inertial frame and he can apply newtons laws there . How would he know if he is in an non-inertial frame?