Aforce that keeps objects in motion from going on forever

It's called "inertia". Inertia keeps an object from going by using friction.

First off, most subjects relating to 'force' is usually physics.
Secondly, There is no force that acts forever.

However, because of the law of inertia (I know, stupid and long name), if you make something move and you just leave it (without anything like oxygen, friction, etc.,. trying to stop the object), it will move forever.

Frictional force exists between your sled and the snow and is directed back up the hill!