Newton’s first law says that if the sum of all the forces acting on a particle is zero then a static particle continues to remain at rest and a moving one continues moving with a constant velocity along a straight line. That means if no net force acting then acceleration of that particle is zero.
Now the question is: Is this law applicable everywhere, in every frame of reference? Or does it work for a specific type of frame of reference?
Let’s us take an example.
Have you observed hand grips inside a bus or train compartment hanging from the overhead rods? Have you noticed their movement when the velocity of the vehicle is changing abruptly (for clarity notice those which are not gripped by passengers)?
Its weight is acting vertically downwards and the tension in its strap is pulling it upwards with an equal and opposite force.
So if we go by the first law of motion then the hand grip should be in stable condition when there is no external force visible, within the framework of the compartment.
But we often see it swaying when the train moves. Why?
We understand that Newton’s first law is to be checked successfully after we consider and find a few things like
– taking a frame of reference
– and then find out the acting forces on a body or a system present within that framework
– and then check if there is any net force present causing an acceleration of the system with respect to that framework.
But from the behaviour of the swaying handgrips in a vehicle having non-uniform velocity makes us think that there are even some frameworks where this first law of motion is not valid.
So what type of framework is only valid here for the first law to be satisfied?
We can take the earth as a valid framework as its moving uniformly. With respect to it(taking it static) we consider a tree or house on it as a static object. We call this frame of earth as an Inertial framework.
Similarly a uniformly moving train is again a valid framework with respect to the inertial framework of earth.
So we can say that all frames moving uniformly with respect to an Inertial frame are inertial themselves. Newton’s first law nicely fits into these frames.
So to conclude, an inertial frame of reference is a framework in which Newton’s first law of motion is valid. example: a uniformly moving vehicle or something which is not moving at all (with respect to the earth)
A frame in which this law doesn’t hold good is called Non-inertial frame of reference. example: a train moving with acceleration.