It is a riddle.
It is the strongest force,
to resist your force,
and yet it is not a force.
It never accelerates,
but it is always moving.
It never rests, and
it never gets tired.
It has been defined many ways,
but no one can quite capture it.
What is it?
Hugo, I'm sorry I didn't mention yours as the first posted solution that matched mine. Congradulations is indeed in order!
Gravity is clearly a force, and therefore a stretch as a solution. Acceleration is a change of direction and/or speed. Time is not scientifically considered a force, but it is metaphorically. If you should doubt that it is not, then just try to stop it! (Metaphors are allowed in riddles.)
The nature of riddles is to be intentionally misleading from the solution. For this reason some people love them, while others hate them for the same reason. Some imagination is required, whether wild or not is debatable. :)