Frames of reference?

My counsel would be to not waste your time. 

This is the first I’ve ever heard of universal acceleration, but it appears they are trying to bastardize the equivalence principle.

In the equivalence principle, a person standing on earth is in a non-inertial frame of reference, at rest with respect to the gravitating object (earth). This is ‘indistinguishable’ from a normal non-inertial frame.

The problem is that the above is only valid locally in a small region of spacetime (exact at a point) unless in a homogenous field. The only way the earth can have a homogenous field - and be flat - is to be infinite. Otherwise, you WILL see geodesic deviation over time along the radial lines of a round earth.

But again, don’t bother. Flat earthers are ignorant, dumb people who compensate for their inferiority and lack of control by giving themselves ‘special knowledge’ that they think elevates them above educated people. Don’t waste your time - they NEED to believe in these wacky things so you can’t change their beliefs using any kind of reason or facts.

Frames of reference don't exert a force. The deal is that in an inertial frame, anything accelerating wrt to that frame has a force acting on it.  And an inertial frame is essentially one where you let go of a rock and it floats next to your hand. But inertial frames don't play well with gravity.

You are wasting your time btw. Years ago it was free energy and perpetual motion, then anti vax, etc. etc. Flat Earth is just the latest vehicle for Stupidity as a Lifestyle Choice. 

How about a train moving on a smooth track? Passengers feel the same gravity force, whether or not the train is moving. They don't feel any force force forward, backwards, left or right. 

But what does this have to do with showing concepts to a Flat Earther?  You better practice on something easy, like teaching calculus to a cow.