## College Physics (4th Edition)

(a) $v = 0.87~c$ (b) The speed of the transmission relative to the Earth is $c$.
(a) We can find the speed of the transmission relative to the Earth, according to Galilean relativity: $v = c - 0.13~c = 0.87~c$ (b) One of Einstein's postulates is that light (i.e. an electromagnetic wave) travels at the speed $c$ in all inertial frames of reference. Note that a radio wave is an electromagnetic wave. Therefore, the speed of the transmission relative to the Earth is $c$.