Consider an inertial reference frame.

Question

In this coordinate system
1. nothing can accelerate.
2. there is no inertia.
3. objects with no forces acting on them
have no acceleration.
4. inertial mass depends on your elevation.

Answer 1

http://www.freedictionary.org/?Query=inertial%20reference%20frame

And Newtons first law says that it takes a force to change motion (ie, accelerate an object).