when can we say that an object is at rest
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".