The equation m=m
0c/sqrt(c
2-v
2) is not generally valid. It breaks down when m
0=0 and v=c. The more general form is
E
2=p
2c
2-(m
0c
2)
2, or (mc)
2=(mv)
2- (m
0c)
2. These latter forms work fine for massless (m
0=0) particles, giving E=pc.
Mike W.
(published on 10/06/2009)
