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4. The equation mgy for gravitational potential energy is valid only for objects near the surface...

4. The equation mgy for gravitational potential energy is valid only for objects near the surface of a planet. Consider two very large objects of mass m1 and m2, such as stars or planets, whose centers are separated by the large distance r. These two large objects exert gravitational forces on each other.The gravitational potential energy is U = − Gm1m2 r where G = 6.67 × 10−11Nm2/kg2 is the gravitational constant.

(a) Sketch a graph of U versus r. The mathematical difficulty at r = 0 is not a physically significant difficulty because the masses will collide before they get that close together.

(b) What separation r has been chosen as the point of zero potential energy? Does this make sense? Explain.

(c) Two stars are at rest 1.0 × 1014 m apart. This is about 10 times the diameter of the solar system. The first star is the size of our sun, with a mass of 2.0 × 1030 kg and a radius of 7.0 × 108 m. The second star has mass 8.0 × 1030 kg and radius of 11.0 × 108 m. Gravitational forces pull the two stars together. What is the speed of each star at the moment of impact?

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