Inertial frame
From Niserwiki
Inertial frames are the the coordinate frames in which Newton's first law holds. That is, when there is no external force acting on an object, the object is at rest or moving with constant velocity. It then also follows that in the inertial frames the acceleration produced in an object is proportional to the applied force with the constant of proportionality being the mass ( or inertial mass ) of the particle. If a frame is
- moving with constant velocity with respect to an inertial frame
- or it's origin is displaced with respect to an inertial frame
- or is having its axes at an angle with respect to an inertial frame
then the frame is also an inertial frame. This so because the acceleration of an object in the two frames is same ( although the velocities of an object measured in the two frames are not same ). This means that there are infinite set of inertial frames.
The frames which are accelerated with respect to an inertial frame are not inertial because an object which is at rest ( or moving with a constant velocity ) in one frame will appear accelerated in another. So, it will appear that a fictitious force is acting on the object in a non-inertial frame. Coordinate frames attached to accelerating vehicles or rotating coordinate frames ( say Earth itself ) are non-inertial frames.
