Two stars A and B have their surfaces separated by a distance of 1.4 x1012m, as illustrated in Fig. 1.1. 1.4 x 1012m star A star B Fig. 1.1 Point P lies on the line joining the centres of the two stars. The distance x of point P from the surface of star A may be varied. The variation with distance x of the gravitational potential at point P is shown in Fig. 1.2. x/1012m 0.6 0.8 $/108 Jkg -10- -12 -14- -16 Fig. 1.2 B) Using Fig 1.2, state the position of P where the net gravitational force experienced by a test mass is O N. Explain your reasoning. [ 2 ] A rock of mass 180 kg moves along the line joining the centres of the two stars, from star A towards star B. C) Use data from Fig. 1.2 to calculate the change in kinetic energy of the rock when it moves from the point where x = 0.1 x 1012 m to the point where x = 1.2 x 1012 m. State whether this change is an increase or a decrease. [ 3 ] D) At a point where x = 0.1 x 1012 m, the speed of the rock is v. Determine the minimum speed v such that the rock reaches the point where x = 1.2 x 1012 m. [ 3 ]