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Can someone please explain the following statement on page 62 of Weinberg's Vol 1 on QFT:

For a system of particles of typical mass m and typical velocity v, the (..) angular momentum operator is expected to be of order J ~ 1

(I understand the part for P ~ mv, so the "quote" is slightly distorted, intentionally).

Also how is

?K of order 1/v

Thanks in advance!