Editing Mass in special relativity (section)

==Invariant mass==

The [[invariant mass]] is the ratio of [[four-momentum]] (the four-dimensional generalization of [[classical three-dimensional momentum|classical momentum]]) to [[four-velocity]]:<ref>{{Citation
|title=Introduction to relativity
|first1=William D.
|last1=McGlinn
|publisher=JHU Press
|date=2004
|isbn=978-0-8018-7047-7
|page=43
|url=https://books.google.com/books?id=PoDYLk6Ugd8C}} [https://books.google.com/books?id=PoDYLk6Ugd8C&pg=PA43 Extract of page 43]
</ref>
<math display="block">p^\mu = m v^\mu</math>
and is also the ratio of [[four-acceleration]] to [[four-force]] when the rest mass is constant. The four-dimensional form of Newton's second law is:
<math display="block">F^\mu = m A^\mu.</math>