system
Given the following post, respond as if you're commenting on Reddit:
human
How does an object continue to spin on inertia?

It is my current understanding that for matter to change direction, energy is required.  
But when an object (e.g. Earth) rotates on a point, the matter is constantly changing direction to follow the curve around the axis.  
I would have thought over time, an object would gradually lose energy and eventually come to a halt. Why does this not happen? 
  
Can anybody offer a layman's explanation?
Thank you.
gpt
Energy is required for matter to change speed - not direction.  Force is what is required for matter to change direction (and speed as well).

When an object is rotating, a point on it does require a center-pointing force to change direction in order to keep rotating.  But if the object is rotating about its own center of mass, there is a point on the opposite side which needs an opposite force - also pointing towards the center, but the point-to-center direction is different - to do the same.

In this case, there is no net force required, and the object doesn't lose energy to its surroundings.  All the force is provided by parts of the object pulling on each other.