Help with 18,19

18. In a 'thought experiment', Newton devised how a cannonball fired horizontally could orbit the earth. For this to happen, which of the following must occur? A) It can't occur. Nothing fired horizontally can orbit the earth. B) The object travels far enough that the distance it falls is equal to the amount the earth curves away. C) It must be fired from the top of a very high mountain. D) It must exit the atmosphere. 19. What is the length of a pendulum on the surface of the moon if its period on the moon is 8.4 s? (g on the moon is 1.63 m/$2) A) 1.8 m B) 0.95 m C) 0.82 m D) 2.9 m