- #1
moenste
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Homework Statement
The diagram shows a binary star system consisting of two stars each of mass 4 * 1030 kg separated by 2 * 1011 m. The stars rotate about the centre of mass of the system.
(a) (i) Copy the diagram and, on your diagram, label with a letter L a point where the gravitational field strength is zero. Explain why you have chosen this point.
(a) (ii) Determine the gravitational potential at L. (G = 6.7 * 10-11 N m2 kg-2.)
(b) (i) Calculate the force on each star due to the other.
(b) (ii) Calculate the linear speed of each star in the system.
(b) (iii) Determine the period of rotation.
Answers: (a) (ii) -5.36 * 109 J kg-1, (b) (i) 2.68 * 1028 N, (ii) 2.59 * 104 m s-1, (iii) 2.43 * 107 s
2. The attempt at a solution
(a) (i) L should be the middle point between these two stars since the point where gravitational field strength is zero is the point where two gravitational fields cancel out.
(a) (ii) Using the formula: U = - Gm / r = - 6.7 * 10-11 * (4 * 1030 + 4 * 1030) / [(2 * 1011) / 2] = - 5 360 000 000 J kg-1.
(b) (i) F = Gm1m2 / r2 = (6.7 * 10-11 * 4 * 1030 * 4 * 1030) / (2 * 1011)2 = 2.68 * 1028 N.
(b) (ii) v = √Gm1 / 2 / r = √(6.7 * 10-11 * 4 * 1030) / (2 * 1011) / 2) = 51 768.7 m s-1. Doesn't fit the answer, though if we divide this by 2 we get 25 884 m s-1, same as the answer.
(b) (iii) T = 2πr / v = (2 π ((2 * 1011) / 2) / (2.59 * 104) = 24 259 403 s.
Is my logic in (a) (i) correct? What's wrong with (b) (ii)? Other calculations should be right, but if not, what did I miss?