Momentum

In a book I read recently one of the main characters was a career soldier (Dalinar in Oathbringer by Brandon Sanderson). He is constantly thinking about how momentum affects a fight or battle. Once you have the momentum if you can keep it up it means that you will most likely win the fight.
Sometimes I just really don't want to write or work on whatever but often once I start I can get into it and it ends up being a good experience.
However, on occasion it just feels like I'm slugging through the whole thing and it's more or less torture.
In physics if you have a frictionless environment (these are only common in physics text books, not real life) once an object has momentum it continues moving indefinitely. When there is friction or other forces on the object it naturally slows down much faster, but depending on how much velocity and mass it has it can overcome friction (depending on the friction) for sometime.
So to return to momentum in life maybe when just starting doesn't seem enough it is because the task is not frictionless and the velocity of starting isn't enough to keep going.