When the distance between two objects is changing, it's called relative motion.

Here's why:

* Relative means that the motion is considered in relation to something else.

* Motion implies that something is moving.

So, relative motion describes the movement of one object as observed from another object.

Examples:

* A car driving past you on the road. The car's distance from you is changing, so it's in relative motion.

* A ball thrown in the air. The ball's distance from the ground is changing, so it's in relative motion.

* Two planets orbiting a star. Their distances from each other and the star are constantly changing, so they are in relative motion.