18. A particle with a life time of 8 x 106 s created at an altitude of 10 km travels directly towards the earth. If it decays as soon as it reaches the earth, what is its velocity relative to the earth? (Ans. 0.972 c) 19. A space ship is observed to cover 90 m in 5 x 10-7 s. What is the distance travelled and time taken (Ans. 72 m, 4 x 10-7s) as measured by an observer in the space ship?