Rest (physics)

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, as the correct frame of reference is usually implicit and/or provided by context, simply "at rest".

Given an inertial frame of reference, Newton's first law of motion states that an object at rest will remain at rest, while the motion of a moving object will remain unchanged until acted upon by an external force. An object at rest, therefore, can be described as without velocity and acceleration – although, according to relativity, an object is either at rest or in motion relative to other moving objects. 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 addressed by the theory of general relativity.

In reality, there is nothing at absolute rest. For example, Earth's gravitation constantly pulls objects toward its surface, while Earth is one of the objects the Sun constantly pulls towards itself, causing it to orbit the Sun; the Sun, in turn, orbits the center of the Milky Way; and so on.