If gravity isn't a force, then how are forces balanced in the real world?

Question

Consider a simple situation like this- an object is sitting on a table. In classical mechanics, we say that the net force on the object is zero because gravity (treated as a force) and normal reaction force are equal and opposite to each other, and hence, it's acceleration is zero. But according to Einstein's General Theory of Relativity, gravity isn't a force at all, but instead curvature created in spacetime by a massive object, and objects near it tend to move towards it because they are just moving along the geodesic paths in that curved spacetime. So if an object kept on a table gets acted only by the normal reaction force (as gravity ain't a force), how is the net force on it zero?

Answer 1

In General Relativity, you can explain this situation using Einstein's Principle of Equivalence, which is the fundamental postulate of General Relativity. Gravity is not a force, it is rather a fictitious or apparent force that results from an accelerating frame of reference. The situation is equivalent to what you would experience if you were traveling in a spacecraft with a constant acceleration of 9.8 m/s/s in the absence of gravity. In this case, the spacecraft is imparting a force of 9.8 N/kg on the table, and the table is imparting a force of 9.8 N/kg on the object. By Newton's 3rd Law, the object imparts an equal and opposite reaction force onto the table. But all three bodies -- the spacecraft, the table, and the object -- are in fact accelerating. The Principle of Equivalence states that there is no physical experiment (mechanical, electromagnetic, or otherwise) that anyone can do to distinguish this situation from that of an object resting on a table on the surface of the Earth.