# Is inertia actually a property of the force rather than the mass?

I ask this because it occurred to me that the inertial property of mass only actually arises in the context of forces (such as the EM force) as a resistance to their accelerating effect. Inertia plays no role in gravity (which as per General Relativity is not really a force, though does resemble a force in certain respects). Under gravity all bodies of whatever mass are accelerated in proportion to the mass of all the other bodies in the universe, in inverse proportion to the square of the distance that stands between them. The role of mass therefore is not to resist acceleration but to warp space-time (and thus induce acceleration, or rather the apparent acceleration of movement in warped space-time).

Under the EM force, and other forces, however, the amount of acceleration that a body experiences depends on its own mass. This relation is generally taken to be an intrinsic property of mass, a resistance to acceleration. But should it not be better conceived as a feature of the force itself, as an action per unit mass, and thus as an acceleration in inverse proportion to mass? In other words, is what appears to be a resistance to acceleration not actually better understood simply as a consequence of the fact that forces act per unit mass? (Mass here is understood as the “bound energy” of General Relativity which warps space-time.)

This re-conceptualisation of the relation between mass and inertia doesn’t have any physical implications of course. But it does perhaps enable us to understand the origin of the equivalence principle (between inertial and active gravitational mass) in that the inertial properties of mass are re-conceived as a property of the forces that interact with it rather than a kind of bolt-on property of the mass itself. The equivalence principle therefore becomes essentially the observation that forces accelerate mass inversely proportionally, on a per unit basis. They could conceivably follow some other rule, some other relation to mass (like gravity does) - though if they did then physics would be quite different of course.

So my question is, is this right? And if it is, does it tell us anything interesting or useful?

• How can a force have a property? It's not an object, it has no reality unless it acts on something. I am really not sure what it is you are asking. Jul 16, 2014 at 13:37
• Get a spring or some other contraption that can always produce the same amount of force. Use this spring on different masses. Each mass behaves differently. Since force was consistent, the difference in behaviour has nothing to do with the force but the property(s) of the masses. We call that property inertia. Jul 16, 2014 at 14:03
• @gregsan I recorded your excellent comment as a community wiki answer. If you happen by here and want to pick up rep for an excellent answer, do so and ping me- I'll delete the community wiki. Jul 20, 2017 at 2:48
• @WetSavannaAnimalakaRodVance I like it this way! cheers... Aug 10, 2017 at 15:09