# Inertial and non inertial frames of references

I've heard from a physics professor that there's no stationary platform to observe and analyze a body in motion.Why did he mention that? Is it because even seemingly stationary objects like a parked car on a road moves along with the earth's rotation?

As per my intuition there is a stationary platform.If you or your car is at rest then that becomes a stationary platform and you can study the kinematics of other objects.Am i right??

Also what is a frame of reference? Which one is intertial and non inertial?

(I might have not framed the question correctly.appologize for that)

-
Also, a stopped car on the surface of the earth is NOT an inertial reference frame. Near the surface of the earth there is significant acceleration due to gravity. – Colin K Oct 5 '11 at 17:42

To establish a frame of reference, you need a reference object and reference direction.

Which one is inertial is actually a tough question. Newton's First Law defines a inertial frame, in which all free particles stay stationary or move at constant velocity. But "free", or "subject to no force" is ill-defined and impossible: gravitation is universal and you can never know whether the gravitational force of the whole universe combined balance out or not.

In practice, we regard some frames as inertial with enough precision. The Earth is a rough inertial frame (more accurately, with the direction pointing to distant stars), the Sun is better and the Milky Way is even better.

In modern view (that is, in general relativity's view), all frames are equivalent, and any inertial frame is infinitesimally small. This view solves many logic difficulties with the classical idea of privileged sets of frames of reference.

-

There's no distinguished stationary platform, such that no others moving with respect to it could claim to be stationary. Another way to put it is, any inertial frame, as far as it's concerned, can claim to be stationary, but nobody on another frame has to agree.

"inertial" only means "not accelerating" or "not having a net force acting on it".

-