20. In frame S' a particle moves with a speed of 2.4 x 10 m/s at an angle of 30 to the X'-axis. If S' is travelling at a speed of 1.8 x 10% m/s along the positive X - X'-axis relative to frame S, find the velocity of the particle (magnitude and direction) as observed by an observer in S. (Ans. v = 2.823 x 10 m/s making an angle of 13.9 with X-axis) 21. Show that + y? + z? - c? = x2 + y2 + z2 - c??. 22. A space traveller desires to go to a star 5 light years away. What should be the uniform velocity of his rocket if he is to reach there in one year as measured by clocks at rest on the rocket? Neglect the time of acceleration etc. (Ans. v 0.98 c)