, . XIX , ,
m = , (β = υ / c) (5.10)
0 - , . . , , ; - ; - , υ. , , .
= (mυ) (5.11)
, .
,
, (5.12)
. (5.13)
, (5.11) . : , (5.12). , (5.13) , , . , , . , .
: , . . .
(5.10), (5.12) (5.13) , , , (5.13) . , () υ << c. υ << c. , - , ( ).
(5.10) . .