In Modern Robotics by Kevin Lynch, there is a term "stationary" frame, but this is never defined. Googling shows that this is synonymous with inertial frame.
Apparently inertial frame means rigidly attaching an accelerometer to a frame would show that the frame has 0 acceleration.
However, in Modern Robotics, it says body frames, which are attached to moving objects, are also inertial frames. How is this possible? If the body frame is attached to a baseball bat, and our robot swings the bat, won't the frame undergo acceleration?