Motion is relative

Motion

 An object is said to be in motion if its position changes continuously with respect to a fixed reference point called origin.

Examples:

(a) The position of a moving train changes with respect to platform (or tracks, trees etc)

(b) The position of flying airplane changes with respect to buildings (or trees, poles etc)

(c) The position of a running athlete changes with respect to marks on the tracks.

If the position of an object does not change with respect to a fixed point (known as origin or reference point) then the object is said to be at rest.

Examples:

(a) The position of walls of a classroom does not change with respect to students and teacher.

(b) The position of a stationary truck does not change with respect to ground (or pole, trees etc)

Motion is relative

The description of motion is different for different observers.

(a) Consider two passengers P1 and P2 sitting in a moving bus while a person P3 is on ground. If we study the motion of P2 alone then following will be the observations of P1 and P3.

P1(inside bus) : P2 is at rest.

P3 (on ground): P2 is in motion.

 (b) The walls of a classroom are at rest with respect to students and teacher inside the class but the same walls appears to be in motion with respect to an astronaut in space.