Special relativity is that being inside a plane going at a constant speed is like being stationary, because time and space change to keep it so. General relativity extends special relativity to explain the acceleration of gravity by changes in space and time as well.
QR5.4.1. Free-fall is Acceleration
QR5.4.2. The Gravity Gradient
QR5.4.3. Unstoppable
QR5.4.4. Gravity Curves Space
QR5.4.5. Gravity Slows Time
QR5.4.6. Black Holes