You have to understand that momentum is not a physical law. It is just an entity to enhance our intuition. read more
MOMENTUM doesn't have its own unit because it is not a fundamental property like velocity or mass. As we know velocity is the product of mass and displacement of a body . Now if we try to relate both we will get a unit for that which is m/s. so if you are sensible than try to think that sensible person will require a unique unit for it . read more
There are only so many fundamental units, then you have to move on to derived units. BTW, momentum and impulse seem to be two names for the same quantity. You can always “name” a derive unit — they are all about. read more
Fourth, they are not mass or momentum, it has its own property. Even though the Inertia has the same unit as mass, the concept is different. The Moment of Inertia does not even have the same unit as momentum. read more