1. The problem statement, all variables and given/known data A projectile is fired from a canon at a velocity of 0.4c at an elevation angle of 30° to the X-axis. An observor is moving at a velocity of 0.1c relative to the canon along the X direction. (a) At what angle does the observor measure the projectile to be fired? (b) What is the projectile’s speed relative to the observor? 2. Relevant equations 3. The attempt at a solution Am I correct in thinking that I use the same method of Lorentz vector subtraction as shown near the bottom of the linked-to webpage below? http://www.relativitycalculator.com/addition_relativistic_velocities.shtml i.e. the section entitled, "What, therefore, would r & θ appear to be to an observor in relatively moving S'?" Example 1 Or does anyone suggest another (simpler?) method? Thank you.