, it is not always correct to state that the velocity of the bullet relatively to us on the embankment is
. This statement is true only when the velocities of the train and the bullet lie along the same straight line. If we neglect to specify this restriction, our statement is incorrect. In Einstein’s theory the problem is very similar.
If a Galilean frame moves away from us with a speed
and if, with respect to this frame and in the direction of its motion, a bullet speeds with a velocity
, the resultant speed of the bullet might be