If the Riemann variants are known, how can we compute the mass motio

1.	If the Riemann variants are known, how can we compute the mass motion velocity u?
A.	( frac {J_++J_-}{2} )
B.	( frac {J_+-J_-}{2} )
C.	( frac {J_+ J_-}{3} )
D.	( frac {J_+ J_-}{6} )
Answer» B. ( frac {J_+-J_-}{2} )

Discussion