Physics
Proper Acceleration
Quick fact
An astronaut in a freely orbiting spacecraft experiences zero proper acceleration even though their coordinate acceleration (relative to Earth) is about 9.8 m/s²—the same value as Earth's surface gravity.
Why this is interesting
You've felt acceleration in a car pushing you into the seat, but what does acceleration really mean when space and time themselves can warp? And why do astronauts float even though they are speeding around Earth?