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The equivalence between inertia and gravity cannot explain tidal effects – it cannot explain variations in the gravitational field. [10] For that, a theory is needed which describes the way that matter (such as the large mass of the Earth) affects the inertial environment around it.
The equivalence principle is the hypothesis that the observed equivalence of gravitational and inertial mass is a consequence of nature. The weak form, known for centuries, relates to masses of any composition in free fall taking the same trajectories and landing at identical times.
A common misconception occurs between centre of mass and centre of gravity.They are defined in similar ways but are not exactly the same quantity. Centre of mass is the mathematical description of placing all the mass in the region considered to one position, centre of gravity is a real physical quantity, the point of a body where the gravitational force acts.
The problem of quantum gravity and the question of the reality of spacetime singularities remain open. [211] Observational data that is taken as evidence for dark energy and dark matter could indicate the need for new physics. [212] Even taken as is, general relativity is rich with possibilities for further exploration.
The n-body problem is an ancient, classical problem [19] of predicting the individual motions of a group of celestial objects interacting with each other gravitationally. Solving this problem – from the time of the Greeks and on – has been motivated by the desire to understand the motions of the Sun, planets and the visible stars.
Examples of fictitious forces are the centrifugal force and the Coriolis force in rotating reference frames. To apply the Newtonian definition of an inertial frame, the understanding of separation between "fictitious" forces and "real" forces must be made clear. For example, consider a stationary object in an inertial frame.
For gravitation, the relationship between Newton's theory of gravity and general relativity is governed by the correspondence principle: General relativity must produce the same results as gravity does for the cases where Newtonian physics has been shown to be accurate.
Inertia is the natural tendency of objects in motion to stay in motion and objects at rest to stay at rest, unless a force causes the velocity to change. It is one of the fundamental principles in classical physics, and described by Isaac Newton in his first law of motion (also known as The Principle of Inertia). [1]