Caption: An animation dynamically illustrating rotating frame (attached to a DISK) whose center is at rest relative to a simple EXTERNAL inertial frame (i.e., an EXTERNAL inertial frame NOT rotating relative to the observable universe).
The ball in the animation is sliding frictionlessly on the DISK and NO ORDINARY forces are acting on it at all.
The left-hand panel gives the EXTERNAL inertial frame perspective and the right-hand panel gives the rotating frame perspective.
Below we give an introduction to the basics of rotating frames. Our discussion mostly assumes the classical limit where Newtonian physics applies and relativistic effects are vanishingly small. Occasionally, relativistic effects are mentioned, but a full discussion of them is beyond our scope.
Features:
The non-inertial frame can be converted into an inertial frame by the introduction of the simple inertial force "-ma" where "m" is the mass of any object under consideration and "a" is the uniform acceleration of the simple non-inertial frame.
Inertial forces are body forces that act equally per unit mass on all bits of a body.
So if a body does NOT resist inertial forces, it suffers NO deformation/strain.
In the classical limit, one can view the introduction of inertial forces as way of generalizing Newton's laws of motion to non-inertial frames.
However, the perspective of the conversion of non-inertial frames to inertial frames seems more useful to yours truly. This is because the conversion to an inertial frame is NOT just a trick. An axiom of general relativity is that almost all physical laws are referenced to inertial frames whether they are simple inertial frames or converted inertial frames. Thus, there is a fundamental likeness of all inertial frames.
General relativity itself is NOT referenced to inertial frames and, in fact, tells us what they are.
One can quibble about whether there are other physical laws NOT referenced to inertial frames, but yours truly thinks the quibbling is a matter of perspective or may amount to saying you are NOT using inertial frames in some definitional sense when effectively you are using them.
However, by convention, a rotating frame is considered one non-inertial frame. It certainly is one reference frame.
But first note that to avoid tedious and unenlightening generality, we will limit our discussion to rotating frames where the rotation axis does NOT have axial precession relative to the observable universe and is at rest relative to an EXTERNAL inertial frame. We also limit ourselves to when rotating frames have constant angular velocities. These limitations can all be relaxed if one needs to.
The animation conforms to our limitations.
A extreme example of the kind of rotating frame we are NOT discussing is one rotating relative to another rotating frame, but NOT rotating relative to the observable universe. Such tricky cases have their interest, but are finicky to discuss.
Recall Newton's laws of motion are referenced to inertial frames although this essential fact is often omitted in high-school presentations.
But, as aforesaid, we can convert non-inertial frames to inertial frame by introducing inertial forces.
The main reason the ball follows a curved path in the animation is the Coriolis force since the ball has velocity relative to the rotating frame of the DISK.
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