19 September 2020 56 5K Report

I am a novice that needs help with a topic that probably seems elementary to more experienced people. The case considered is a mass M sitting on Earth’s surface. Throughout this discussion, “stationary” means stationary relative to Earth’s surface at the location of M. Newtonian mechanics applied to a stationary coordinate system sees the normal force on M (the force that Earth’s surface exerts on M) and the gravitational force as real (as opposed to pseudo) forces that balance each other so M does not accelerate. Now consider a coordinate system that is initially stationary but accelerating downwards in accordance with gravity. We call this the falling coordinate system. General relativity calls this the inertial coordinate system so the real forces are the ones seen in this coordinate system. Only the normal force is real and this is the only force seen in this coordinate system. In this view, the gravitational force is a pseudo force needed by a non-inertial coordinate system (the stationary coordinate system) in order to calculate correct answers. The falling coordinate system sees M to be accelerating upwards in accordance with the normal force so M is gaining kinetic energy as seen in this coordinate system. An inertial coordinate system should have energy conservation laws so the gain in kinetic energy must be coming from somewhere. Where does it come from?

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