What are accelerative and inertial frames of references?

Answer 1

An inertial frame is a system in which none of the objects are changing speed or direction with respect to the others. It can be either 'stationary' (if you ignore the fact that there is no absolute, universal frame of reference) or moving with a constant velocity. Special relativity describes the effects of motion in inertial frames and is actually fairly simple.

Accelerating frames of reference are those which are undergoing either a change in speed or direction or both. When you are falling down your speed is changing and when you are orbiting the earth your direction is changing, for example. Describing motion in accelerating frames is far more complex, such as general relativity (which reduces to special relativity when acceleration is zero).

Answer 2

When Einstein first worked on his relativity theories, he recognized that forces could be mainifest by either acceleration or by the bend in space due to massive mass (like planets). A favorite example of this given by those who tried to simplify this concept for us lay people was the person in the elevator.

If all other senses were neutral one could not tell if the "weight" of a person inside an elevator came from gravity or from the elevator accelerating upward. Looking around the elevator, the person inside would have no clues as to the source of the force on his soles. The elevator confines are that person's frame of reference in this case.

If the source of the force is gravity (the bend in space), then that elevator would be an inertial frame of reference. If the force is due to upward acceleration, then that elevator would be a fictituous gravity (your "accelerative") frame of reference. Check this out:

"Equivalence of inertial reference frames
A fundamental principle of all physics is the equivalence of inertial reference frames. In practical terms, this equivalence means that scientists living inside an enclosed box moving uniformly cannot detect their motion by any experiment done exclusively inside the box. [They are surrounded by the inertial frame where only gravity has an effect.]

By contrast, bodies are subject to so-called fictitious forces in non-inertial reference frames; that is, forces that result from the acceleration of the reference frame itself and not from any physical force acting on the body. Examples of fictitious forces are the centrifugal force and the Coriolis force in rotating reference frames. Therefore, scientists living inside a box that is being rotated or otherwise accelerated can measure their acceleration by observing the fictitious forces on bodies inside the box." [See source.]

By the way, the concept of a "reference frame" is critical to understanding relativity. In fact, the term "relativity" was coined because those theories were exposited relative to specific frameworks.

For example, when you see by your speedometer that you are driving 85 mph (way over the speed limit), that speed is relative to the Earth you are driving on. The Earth's ground is the framework, the frame of reference, for specifying the car's speed.

If you were to calculate the speed of that car relative to the Sun as a fixed point (the new frame of reference), you'd find that car going a heck of a lot faster than 85 mph. Why? because now you have to add in the rotation of the Earth and its revolution around the Sun. The rotation alone is another 1,000 mph or so.