How Devices Measure Gravity: Primarily using an Accelerometer: Measures *proper* acceleration (acceleration relative to a free-falling frame). Based on tiny mechanical structures (MEMS - Micro-Electro-Mechanical Systems) that deflect under acceleration. In a stationary device on a surface, the accelerometer measures the upward force exerted by the device's support (like a table) counteracting gravity, which is equal in magnitude and opposite in direction to the gravitational acceleration. Locally, gravity is indistinguishable from acceleration. An observer in a windowless elevator cannot tell whether they are at rest in a gravitational field or accelerating in gravity-free space. This insight was crucial for Einstein's development of GR, suggesting that gravity could be understood as a geometric effect rather than a force. GRAVITY SHOULD NOT DEPEND ON HUMAN SUBJECTIVE PERCEPTION! GRAVITY = ACCELERATION = INERTIA Newtonian gravity is an exceptionally accurate approximation of General Relativity in the limit of weak gravitational fields (where spacetime is only slightly curved) and at speeds much less than the speed of light. Pragmatism: For most everyday applications, and even for significant engineering feats like launching rockets into Earth orbit or calculating trajectories within the solar system (away from strong sources like the Sun or Jupiter for maximum precision), Newtonian mechanics is significantly simpler computationally and provides sufficient accuracy. Using the full machinery of GR would be unnecessarily complex. Using Newtonian gravity for calculating a baseball's trajectory is pragmatic; using GR for GPS satellite timing (X.101) and cosmology (X.103) is essential because the relativistic effects are significant and measurable. ==relative to what? our perception/observation? Relative to the measurement (GPS measures what is happening, rocket launch is a model of what will happen)?== Gravity doesn't exist without movement? (acceleration and inertia)