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If inertia is resistance to change then negative inertia would be promotion of change? I really dont know, the idea of negative inertia seems wildly counterintu

If inertia is resistance to change then negative inertia would be promotion of change? I really dont know, the idea of negative inertia seems wildly counterintuitive.

thaumasiotes 8y ago
Inertia isn't "resistance to change". It's defined by Newton's second law, F = ma. Force and acceleration are directional vectors, and mass is scalar. In the form of the equation favored by my high school physics teacher, a = F/m, we see that when a force is exerted on an object with positive mass, the object will accelerate in the same direction as the force, in inverse proportion to its own mass. But it's very easy to put a negative number in for the mass, and then you see that such an object would accelerate in response to force just as much as an object of equal positive mass, but in the opposite direction.
- wallace_f 8y ago
  Thanks, that's obviously a much better way to think about it. However, you're wrong in at least the typical physics definition of inertia found in dictionaries is in fact, "resistance of change (velocity, acceleration, momentum)." In that same framework, the maths definition of inertia is also momentum = mv and a google search confirms this. What would negative momentum even look like? Because that's not just movement in the opposite direction.
  - thaumasiotes 8y ago
    Velocity, the v in the definition of momentum, is also a vector, so yes negative momentum is just momentum in the opposite direction from positive momentum, whatever direction that may be. Inertia is generally not defined as being momentum, because an object's momentum in any direction increases along with its velocity in that direction, while we like to think of inertia as being determined solely by mass. https://en.wikipedia.org/wiki/Inertia https://en.wikipedia.org/wiki/Inertia begins "Inertia is the resistance, of any physical object, to any change in its velocity". That's specifically resistance to acceleration ("change in velocity"), not resistance to the concept of change in general. (It does go on: "In common usage, the term "inertia" may refer to an object's "amount of resistance to change in velocity" (which is quantified by its mass), or sometimes to its momentum, depending on the context.")
    - wallace_f 8y ago
      But that's what I just said: resistance to change in v, a, p are the dictionary definitions I found on Google. You're right about v. I thought a negative velocity was equally absurd, but it looks like it's defined as a vector which makes sense of course. The last time I took a proper physics course was in high school. How embarassing though, as I've read and watched more popular science/physics material than most people, probably. Hah.
  - Retra 8y ago
    Dictionaries are not written by physicists.

If inertia is resistance to change then negative inertia would be promotion of change? I really dont know, the idea of negative inertia seems wildly counterintuitive.

thaumasiotes 8y ago

Inertia isn't "resistance to change". It's defined by Newton's second law, F = ma. Force and acceleration are directional vectors, and mass is scalar. In the form of the equation favored by my high school physics teacher, a = F/m, we see that when a force is exerted on an object with positive mass, the object will accelerate in the same direction as the force, in inverse proportion to its own mass. But it's very easy to put a negative number in for the mass, and then you see that such an object would accelerate in response to force just as much as an object of equal positive mass, but in the opposite direction.

wallace_f 8y ago
Thanks, that's obviously a much better way to think about it. However, you're wrong in at least the typical physics definition of inertia found in dictionaries is in fact, "resistance of change (velocity, acceleration, momentum)." In that same framework, the maths definition of inertia is also momentum = mv and a google search confirms this. What would negative momentum even look like? Because that's not just movement in the opposite direction.
- thaumasiotes 8y ago
  Velocity, the v in the definition of momentum, is also a vector, so yes negative momentum is just momentum in the opposite direction from positive momentum, whatever direction that may be. Inertia is generally not defined as being momentum, because an object's momentum in any direction increases along with its velocity in that direction, while we like to think of inertia as being determined solely by mass. https://en.wikipedia.org/wiki/Inertia https://en.wikipedia.org/wiki/Inertia begins "Inertia is the resistance, of any physical object, to any change in its velocity". That's specifically resistance to acceleration ("change in velocity"), not resistance to the concept of change in general. (It does go on: "In common usage, the term "inertia" may refer to an object's "amount of resistance to change in velocity" (which is quantified by its mass), or sometimes to its momentum, depending on the context.")
  - wallace_f 8y ago
    But that's what I just said: resistance to change in v, a, p are the dictionary definitions I found on Google. You're right about v. I thought a negative velocity was equally absurd, but it looks like it's defined as a vector which makes sense of course. The last time I took a proper physics course was in high school. How embarassing though, as I've read and watched more popular science/physics material than most people, probably. Hah.
- Retra 8y ago
  Dictionaries are not written by physicists.