energy conservation of momentum
Fig. 6-1 (bottom). Consider an arbitrary body that is stationary in reference frame S. Let this body emit a pair of equal-energy light-pulses in opposite directions at angle 



ϕ


{\displaystyle \phi }

 with respect to the x-axis. Each pulse has energy 



L

/

2


{\displaystyle L/2}

. Because of conservation of momentum, the body remains stationary in S after emission of the two pulses. Let 




E

0




{\displaystyle E_{0}}

 be the energy of the body before emission of the two pulses and 




E

1




{\displaystyle E_{1}}

 after their emission.
