So inertia is really just resistance. Why can't resistance just be used?
Because inertia is NOT "just" resistance!
"Resistance" in this context is just something that resists motion and causes an object in motion to slow. Common forms of resistance are:
- Friction, which is highest (static friction) until an object it starts moving, when it falls to a lower "kinetic friction" when the object is moving. "Kinetic friction" is reasonably independent of velocity.
Friction can be loosely thought of the force needed to overcome the rubbing together of tiny bumps on the surfaces of objects.
- Fluid resistance, which at low speeds at least is due to the viscosity of the fluid. The distinction between friction and fluid resistance is that fluid resistance is roughly proportional to velocity. It is zero for very low speeds.
Friction and fluid drag are present for even
constant velocities, but "inertia" is what opposes "
changes in velocity".
An object at rest
subject to no forces at all stays at rest.
To increase the velocity requires an acceleration and this requires a
force = mass x acceleration.
Once the object is in motion it will stay in motion unless some force is applied to it.
Now as you say there is
almost nowhere on earth when this is precisely true, but a massive flywheel on very good bearings can keep spinning for hours, but it is still subject to some friction and drag, so it does slow down.
Probably the best practical systems are energy storage systems using high speed (30,000 rpm and faster) in a vacuum, levitated via magnetic bearings.
Drop an object in a near perfect vacuum is as close as we can get to completely removing friction and drag.
But ideally, the principle still holds, an object with no force applied stays at rest or continues moving at a constant velocity.
"Inertia" is a fairly loose term for this resistance to change in velocity, and it the simple case of l
inear motion is simply mass.