Einstein's thought of a person under free-fall and when suspended in free space with no forces acting, will feel the same way. On Walter lewin's lecture on free fall he told free fall is when only gravity force is acting on a body (Newtonian Mechanics).

How is these two thoughts connect, how both the conditions are same, one has a net force acting other has no net force. If we have to design a component/machine to work under these conditions, do we have to consider any forces, if so, which force.

Edit: Improved understanding There is no force of gravity, when a body is in free space, the space-time is flat, in a gravity field its curved, a body placed in both will follow a geodesic path if no external force is acting. This motion of the body in a geodesic path on a curved spacetime is manifested as a body under constant acceleration.

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    $\begingroup$ Hello Azeem. Welcome to PSE! The fact that both these scenarios are the same is the essence of the Principle of Equivalence. When you ask "if we have to design a machine to work under these conditions" what exactly do you mean? What would this machine need to do? $\endgroup$ – joseph h Sep 4 '20 at 5:25
  • $\begingroup$ Re, "...suspended in free space with no forces acting..." I think that choice of words is leading you astray. "Suspended" normally means that some other force is acting on you. (e.g., "suspended by a rope.") There aren't "two thoughts" here. "free fall" and "no forces (other than gravity) acting" both mean exactly the same thing. $\endgroup$ – Solomon Slow Sep 4 '20 at 14:08

What you feel as weight while standing on the ground is not because of gravity but due to the normal contact force applied by the ground on you. When that support is removed you won't feel your weight.

If you are hanging in space with nothing to apply force on you then you are not going to feel your weight.

And When you are falling under gravity with nothing to counteract your fall or say apply the contact force then also you won't feel your weight and that's why we say both the situations as free fall.

So both are related as in one there was no force and in the latter there was no support or contact force. So both the situations we lack the normal contact force.

This is exactly what is exactly explained in your linked video.

  • $\begingroup$ Re, "hanging in space" vs. "falling under gravity." Those situations aren't just "related:" They are the same exact thing. An astronaut on a space walk, floating beside the International Space Station (ISS) is "falling under gravity." The ISS itself (when they aren't firing an engine to correct its orbit) is "falling under gravity." The Earth itself is falling under gravity, and so is our entire solar system, and so is our entire galaxy. There is no place in the universe where you would not be "falling under gravity" if no other force was acting on you. $\endgroup$ – Solomon Slow Sep 4 '20 at 14:03

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