Rest (physics)

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Rest in Physics refers to an object being stationary relative to a particular frame of reference or another object. When the position of a body with respect to its surroundings does not change with time it is said to be at rest. According to the theory of relativity it is said that an object is at rest relative to another. For example, a train decelerates approaching a station and eventually comes to rest alongside the platform. The train can be said to be at rest with respect to the station or just at rest, since in practice we do not need to specify the frame of reference if it is obvious from the context.

Newton's first law of motion states that from an internal frame of reference, an object at rest will remain at rest and an object in motion will remain in motion unless an unbalanced force acts upon it.[1][unreliable source?] Therefore an object at rest can be most simply stated as an object with no velocity and a constant acceleration although, according to Albert Einstein's theory of relativity an object is either at rest or in motion relative to motion around the objects surrounding. The concept of relative rest is closely linked to that of inertial observers and the statement that nothing is at absolute rest is loosely equivalent to stating that there are no frames of reference which are truly inertial. So-called non-inertial observers are dealt with in the theory of general relativity.

In fact, there is nothing at absolute rest. For example, Earth's gravitation is accelerating objects to its surface constantly, the Earth is rotating around the Sun which is rotating around the center of the galaxy and so on.