11. They orbit each other about a common centre of mass , a two body problem. 12. :Balanced external forces means that the object's centre of mass is not accelerating. 13. The two stars take approximately 40 days to complete an orbit around their common centre of mass . 14. If the centre of mass is placed forward of the fins, the rocket will nose dive. 15. Thus the'geometric'and'centre of mass 'is not necessarily going to coincide. 16. Several systems are known to orbit a common centre of mass and are known as binary companions. 17. This is true in general : nonlinear Hartree equations never have an influence on the centre of mass . 18. As there are no horizontal forces on the stick, its centre of mass must fall vertically downwards. 19. This is reversed in running where the centre of mass is at its lowest as the leg is vertical. 20. The axis passes through the reflector's centre of mass , allowing the reflector to be turned easily.
