What is relative motion and how does it affect your ability to describe motion clearly?

Answer 1

Relative motion is the motion of one object from the perspective of another object. If you were to fire bullets out of a machine gun in deep space, the relative motion of any bullet to another would be zero; that is they would all maintain the same distance from each other constantly. Yet relative to the machine gun they'd all be shooting off at high speeds.

This is important to understand because motion can only be defined in a relative way such as this. There is no universal frame of rest, where things are absolutely stopped. Even when you're sitting on the floor you're not truly at rest, because not only is the tectonic plate your home is built upon moving relative to the center of the Earth, but also Earth itself is moving around the sun, the sun around the galactic core, and the galaxy towards the barycenter of the Virgo Supercluster, which itself is presumably careening around the universe in some direction or other.

Another way to describe it is if you were on a blank white flat ground, in a hovercraft that had no indication of it moving, and there was another one beside you, travelling the same speed, it would appear that both of you are still, when in reality you could be moving. If he was travelling 40mph and you were moving 39mph, it would apear you are still while he is moving at a slow speed away from you.

Now if you were heading 20 mph and he was travelling in the opposite direction at the same speed it would appear as if he was moving at 40mph away from you.