A frame of reference is a set of three mutually perpendicular lines (direction or axes) used to specify the position of a particular or an event occurring in space.
"Any frame of reference, in which Newton's laws of motion hold and can describe the dynamics of the moving objects is called an inertial frame of reference."
In accordance with Newton's first law of motion, any system acted upon by zero net external force is an inertial frame of reference. In other words, any unaccelerated frame of reference, i.e., a frame of reference at rest or in uniform motion along a straight line (w.r.t. absolute space) is an inertial frame of reference.
Following are a few examples of inertial frame of reference:
(1) A frame of reference remaining fixed w.r.t. stars in an inertial frame of reference.
(2) A spaceship moving in outer space without spinning and with its engine cut off is also inertial frame of reference.
(3) For practical purpose, a frame of reference fixed to earth can be considered as an inertial frame. Strictly speaking, such a frame of reference is not an inertial frame of reference, because the motion of the earth around the sun is an accelerated motion due to its orbital and rotational motion. However, due to negligibly small effects of rotation and orbital motion, the motion of the earth may be assumed to be uniform and hence a frame of reference fixed to it may be regarded as inertial frame of reference.
The inertial frame of reference is important for the following reasons:
(a) All the fundamental laws of physics have been formulated in respect of inertial frame of reference.
(b) All the fundamental laws of physics can be expressed as to have the same mathematical form in all the inertial frames of reference.
(c) The mathematical and optical experiments performed in an inertial frame in any direction will always yield the same results. It is called isotropic property of the inertial frame of reference.
A frame of reference having accelerated motion is called a non-inertial frame of reference.
In an accelerated frame of reference an object experiences a fictitious force. For example, a bus moving along a circular track is an accelerated frame of reference. It is our experience that the passengers sitting in such a bus are always thrown away from the centre. This force which tends to throw the passengers away from the centre is called centrifugal force and is a fictitious force. The motion of objects in accelerated frame of reference cannot be studied by simply applying Newton's laws of motion.
