The word dynamics has to do with change or motion. In music, it relates to the volume of what is being performed.
Musical dynamics offer contrast to make music more interesting: not everything is loud, and not everything is soft. Change is interesting.
Rhythm is all about the duration and frequency of performed sounds or silences. (Notes and rests)
In other words...
How long should this note last? That's asking about its duration.
How many of these notes will I play in a row? That has to do with frequency.
Rhythmic symbols show how long sounds last in a relative way, and they help you plan what a sequence of notes will sound like.
Be sure you can identify these rhythms.