Physics-
General
Easy
Question
Two stars bound together by gravity orbit each other because of their mutual attraction. Such a pair of stars is referred to as a binary star system. One type of binary system is that of a black hole and a companion star. The black hole is a star that has collapsed on itself and is so massive that not even light rays can escape its gravitational pull. Therefore, when describing the relative motion of a black hole and a companion star, the motion of the black hole can be assumed negligible compared to that of the companion. The orbit of the companion star is either elliptical with the black hole at one of the foci or circular with the black hole at the centre. The gravitational potential energy is given by U = – Gm M/r, where G is the universal gravitational constant, m is the mass of the companion star, M is the mass of the black hole, and r is the distance between the centre of the companion star and the centre of the black hole. Since the gravitational force is conservative, the companion star’s total mechanical energy is a constant of the motion. Because of the periodic nature of the orbit, there is a simple relation between the average kinetic energy 
of the companion star and its average potential energy < U>. In particular, 
Two special points along the orbit are singled out by astronomers. Perigee is the point at which the companion star is closest to the black hole, and apogee is the point at which it is furthest from the black hole.
Which of the following prevents the companion star from leaving its orbit and falling into the black hole?
- The centripetal force
- The gravitational force
- The companion star’s potential energy
- the companion star’s kinetic energy
The correct answer is: The centripetal force
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