| MadSci Network: Physics |
In fact, I have never seen "J" used to represent impulse, but rather, usually, "I". See, for instance, this definition for Impulse.
Sometimes there is an historical reason for the letter usually associated with a variable in physics (and other sciences), but not always. Tradition plays a role, also, since new generations of scientists must be able to understand the work of their predecessors.
There are some previous answers in our archives which discuss some of the common variables used in physics. You can find them by using our search engine to search on things like "why p variable" or "why i variable" or "why j variable", etc.
John Link, MadSci Physicist
Try the links in the MadSci Library for more information on Physics.