“The surface acceleration of Earth is 9.8 meters/sec2”

Question

A free falling body doesn’t experience gravity but is moving relative to a stationary observer. This observer experiences gravity. – check.

An accelerating body experiences “gravity” and a stationary body observed by the accelerating body appears to be in motion. – check.

There’s a Veritasium video that covers this: [Why Gravity is NOT a Force](https://youtu.be/XRr1kaXKBsU).

Here’s where it gets a bit mind-bendy though. If the surface of the earth is stationary, why is it pushing up on our feet like the floor of an accelerating rocket would? Is it because although it is stationary relative to our space, it *is* moving through TIME?

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Answer 1

The idea that gravity isn’t really a force is a pretty high-level concept that I wouldn’t recommend for an ELI5 understanding of gravity. But if you do want to run with it, here’s a simple (and true-ish, before you get too deep into general relativity) statement to help get your head around it:

The Earth’s orbit around the Sun is a straight line.

Not in space as we perceive it, of course, but in spacetime, it is (sort of) a straight line. This is (sort of) true because of the way spacetime is curved near big heavy objects like the Sun. Things travel in straight lines when there are no forces acting on them, so the Earth orbits the Sun because there aren’t any forces acting on it (remember: gravity isn’t a force in this setup). Note, though, that the Earth’s orbit is only straight in spacetime, and only if it goes at the specific speed that the Earth is moving at. A slower or faster orbit in the same physical path would not be a straight line in spacetime.

Once you’ve got your head around that, you might be able to see how a satellite orbiting the Earth is moving in a straight line, and a bowling ball dropped from the Leaning Tower of Pisa is moving in a straight line (if we ignore air resistance in both cases), but a human standing still on the surface of the Earth is *not* moving in a straight line. Therefore, the human must have a force pushing on them. That force, by the way, is the force of the ground trying to expand against the pressure of the human and the dirt being jammed together by proximity. It’s typically called the “normal force” in Newtonian physics, because it acts “normal” (perpendicular) to the surface of the ground.