Inertia for your head.
For example, if you procrastinate now, you will procrastinate tomorrow and the next day until something knocks you up and out of it.
For example, if you act now, you will continue to act until exhaustion or boredom stop you.
If you are miserable, you will continue to be sad until you are made to be happy, if you are elated, it will continue until you are made sad.
This requires two more concepts to make sense: a ground state, and a type of gravity. The ground state is neutral emotion/action. Through success, exhaustion, renewal of want, and shit happening, you are returned to this ground state.
Additionally, there may be a concept of mass in/of thought. In physics, the mass must be exceeded before action is achieved. What makes mass for this system?
Maybe it is you, and you must abandon yourself to move.