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 Originally Posted by OngBonga
Huh? There certainly are inertial forces. Coriolis, centrifugal, centripetal, and the biggie... gravity, aka know as "fictitious forces" because they only emerge in a non-inertial frame of reference, that is an accelerating or rotating frame of reference. But doesn't literally everything with mass (except maybe black holes) exist in a non-inertial frame of reference? Assuming it's not a singularity, then it has gravitational interactions with itself. So everything with mass that isn't a black hole exists in a non-inertial frame of reference, and therefore experiences inertial forces.
Inertia is indeed basically mass, but it's better defined imo in layman's terms as "resistance to a change in state of motion". Linking that concept to mass isn't easy, but we all understand that an object with mass requires a force to set in motion so the intuition is just about there.
It's the difference between colloquial language and scientific jargon.
In physics, there are no inertial forces. What a colloquial speaker would call an inertial force is not a force at all. It's just Newton's Laws of motion. In an inertial reference frame, a moving object will continue moving in a straight line at constant speed if and only if the vector sum of forces acting on it is 0 N. A *lack of force* is responsible for inertial motion.
The phrase inertial force comes from a misunderstanding of the laws of motion. It's based on an assumption that an object with no forces acting on it will be still. To put it another way, it assumes that the reason for motion is forces. This seems pretty true in a macroscopic world with friction causing most movements to slow and stop. If you don't keep applying a force, it will stop. It fails to appreciate that friction is a force, and that the stopping of the motion isn't due to a lack of forces, but due to a stopping force.
Per your question on reference frames: Things aren't in any given reference frame, inertial or otherwise. A reference frame is a perspective. From some perspectives, Newton's Laws hold, and those perspectives are called inertial reference frames. From other perspectives, Newton's Laws are simply not true*; these are called non-inertial reference frames.
There is no universally "prime" reference frame from which we could say an object is "truly" at rest in any sense. The laws of physics we have simply don't care whether or not you're moving. We have the mathematical framework to assert a reference frame that is helpful for a human to understand the relationships and consequences of interactions.
*Better to say they don't apply than to say they're not true. Each of Newton's laws of motion contains the phrase, "in an inertial reference frame." So if you're not in an inertial frame, it's not exactly that Newton's Laws are untrue, just that they specifically don't apply.
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