The title says "momentum" maybe that attributes to the perfume ilya gave Misha normally all the relationships have momentums first relationship was a movie second was a camera so on and so fourth.
"the force that keeps an object moving"--all of these people are moving/growing/changing in these relationships. It's the relationship that causes them to keep moving, growing, making progress, they just manifest differently depending on the talents/career/etc of the characters and the settings. The photographer was always taking pictures, even before the relationship. But being in that situation, with those people, taught him how to be a better photographer, for example, and become successful professionally.
The fact that it had BL in it had me 'clickin.