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Question 1 (10 marks) (a) (4 marks) Recall that the gravitational potential energy for two masses is Ug-GMm Use this fact to show that the virial theorem holds for a mass m, executing uniform circular motion about a much larger mass M (b) (3 marks) The potential energy of a collection of N particles each of mass m in a region with radius R can be written as Use this expression to derive a mass estimate for the virialized system in terms of its radius and a root mean square velocity of orbits, v. (c) (1 mark) Suppose the orbits are randomly distributed isotropically in all directions with a velocity dispersion of σ. What is the mass estimate in terms of the size of the system and this velocity dispersion? (d) (2 marks) We observe another system which is a spiral galaxy with tidal streams and a warped disc. May we confidently apply the virial theorem in this case? Why or why not?

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