Motion in Straight Line: Rest & Motion

Meaning of Rest and Motion

Rest and motion are the two states of a body.

Rest: A body is said to be rest if it does not change its position with time w.r.t. its surroundings.  Ex. a book lying in air, a man sitting on desk etc.

Motion: A body is said to be in motion if it changes its position with time w.r.t. its surroundings. Ex. a bird flying in air, a man walking on road etc.

The world, and everything in it, moves.  Even seemingly stationary things, such as roadway, move with Earth's rotation, Earth's orbit around the Sun, the sun's orbit around the centre of the Milky Way galaxy, the galaxy's migration relative to other galaxies.

Rest and motion are relative

It means that an object can be rest w.r.t. one object and at the same time can be in motion w.r.t. another object

Example (i) When we are sitting in a moving train compartment, we are at rest w.r.t. the other passenger sitting in it.  But to an observer outside the compartment, we appear to be moving.

(ii) A person sitting in his house is at rest w.r.t. earth but is in motion w.r.t. another planet.

From the above two examples, it is clear that whether the position if of rest or of uniform motion, it is only relative i.e. depends only upon the position of the observer.  Thus, there is no meaning of rest or motion without observer.

Meaning of Absolute Rest and Absolute Motion

Absolute rest is complete absence of motion.  To determine this, we require some 'fixed reference object'.  If a body is at rest w.r.t. this fixed reference object, it will have absolute rest.  But there is no fixed object in this universe.  All heavenly bodies are moving w.r.t. one another.  Thus, absolute rest is beyond our imagination.

Similarly, absolute motion is also practically not possible as we do not have any reference point which is absolutely fixed in space.