As a noun:
* It refers to the mass of a moving object multiplied by its velocity.
* It can also be used figuratively to describe a force or driving power that is gaining strength.
Examples:
* "The car gained momentum as it sped down the hill." (noun, scientific meaning)
* "The team had a lot of momentum going into the playoffs." (noun, figurative meaning)
As an adjective:
* It describes something that has momentum, often in a figurative sense.
Example:
* "The candidate had a momentum-building campaign."
So, the part of speech depends on how the word is used in a sentence.