# Inertial Reference System

Hi All,

I'm a bit confused about the definition of an internal reference system!

I know that a system can be considered inertial if there is no acceleration relative to the background stars. In other words, a system moving at constant velocity.

What about the rotation of such a system? If it rotates with a constant angular velocity, it it still considered an inertial syste?

Bob

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No, it is not. So every frame of reference on earth is not absolutely inertial. An inertial reference frame must not have any acceleration exerting on.

Danger
Gold Member
It might be dependant upon what definition you're going by. In the strictest sense, your personal frame is the only relevant one. If you are relying upon a GPS unit, then that unit's frame is the relevant one. If you have none of the above, then just nuke 'em until they glow, then shoot 'em in the dark.

http://quick-help.net/Inertial-Reference-System-Definition [Broken]

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Shooting Star
Homework Helper
I'm a bit confused about the definition of an internal reference system!
An idealised frame of reference in which Newton's 1st law of motion is valid. If a body is not acted upon by any force, it will continue to move with uniform velocity wrt that frame (or stay at rest).

I know that a system can be considered inertial if there is no acceleration relative to the background stars. In other words, a system moving at constant velocity.
That is a workable definition over (astronomcally) small distances and time.

What about the rotation of such a system? If it rotates with a constant angular velocity, it it still considered an inertial syste?
Definitely not, as Pixel01 pointed out.

It might be dependant upon what definition you're going by. In the strictest sense, your personal frame is the only relevant one. If you are relying upon a GPS unit, then that unit's frame is the relevant one. If you have none of the above, then just nuke 'em until they glow, then shoot 'em in the dark.
Ulg! Zhug?

Danger
Gold Member
Ulg! Zhug?
Apparently my lack of formal education is showing.
From my reading about relativity (mostly SciAm, since I can't follow math), my understanding was that an inertial frame is one in which nothing moves relative to anything else. For instance, if I'm walking along, I'm not actually moving, nor is whatever I'm carrying; everything else is moving relative to me.

D H
Staff Emeritus
An inertial reference frame is characterized by two things: An origin, and a set of axes. Suppose there does exist some true inertial reference frame. We want to test whether some other reference frame is inertial. Examining the translational behavior of the origin and the rotational behavior of the axes of this other frame will determine if this other frame is inertial. If the origin of this other frame is accelerating with respect to the origin of the true inertial frame, an observer fixed to this other frame will see objects not subject to external forces undergo acceleration. Similarly, If the frame's axes are rotating with respect to the inertial frame, our observer will see objects that are not subject to any external forces follow a curved path.

At one of my previous jobs I worked on devices that were flew from the middle of the ocean and ended their flight, rather spectacularly, over land. During a discussion with my supervisor, PhD in physics, he nonchalantly claimed "there is no such thing as an inertial reference frame." My reply was along the lines of "what???" "Name one."

The best we can do is to find something close to an inertial reference frame. Our best guess as far as axes is the International Celestial Reference Frame (ICRF). The US Naval Observatory website on the ICRF is http://rorf.usno.navy.mil/ICRF/" [Broken].

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Dale
Mentor
An inertial reference frame is characterized by two things: An origin, and a set of axes.
Any reference frame, inertial or not, has an origin and a set of axes.

Suppose there does exist some true inertial reference frame. We want to test whether some other reference frame is inertial. Examining the translational behavior of the origin and the rotational behavior of the axes of this other frame will determine if this other frame is inertial. If the origin of this other frame is accelerating with respect to the origin of the true inertial frame, an observer fixed to this other frame will see objects not subject to external forces undergo acceleration. Similarly, If the frame's axes are rotating with respect to the inertial frame, our observer will see objects that are not subject to any external forces follow a curved path.
Although you can define inertial-ness this way I don't think that you should. How do you know that your "true inertial frame" is in fact inertial? This is unnecessarily complicated since you can determine the inertial-ness of a reference frame without comparing to any other reference frame. In fact, it is necessary to do so for your "true inertial frame" anyway, so you may as well discard the concept altogether.

At one of my previous jobs I worked on devices that were flew from the middle of the ocean and ended their flight, rather spectacularly, over land. During a discussion with my supervisor, PhD in physics, he nonchalantly claimed "there is no such thing as an inertial reference frame." My reply was along the lines of "what???" "Name one."

The best we can do is to find something close to an inertial reference frame. Our best guess as far as axes is the International Celestial Reference Frame (ICRF). The US Naval Observatory website on the ICRF is http://rorf.usno.navy.mil/ICRF/" [Broken].
This is actually more along the lines of how I would define an inertial reference frame. An inertial frame is a coordinate system (origin and axes) in which an ideal inertial guidance device (like what you are describing above) at rest in the frame will register no acceleration and no rotation. This makes it clear that the term "inertial" is a physical description.

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Shooting Star
Homework Helper
If you have none of the above, then just nuke 'em until they glow, then shoot 'em in the dark.
Well, I thought you were an alien about to nuke us. So I tried to ask hey dude what's up?

Danger
Gold Member
Well, I thought you were an alien about to nuke us.
Ummm... er... no. I'm here merely to observe... :uhh: