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When the magnetic domains in a material can be aligned, but eventually drift out of alignment, the material is

Two identical stars with mass m orbit around their center of mass. each orbit is circular and has radius r, so that the two stars are always on opposite sides of the circle. (a) find the gravitational force of one star on the other. (b) find the orbital speed of each star and the period of the orbit. (c) how much energy would be required to separate the two stars to infinity?

What would be the radius of the earth if it had its actual mass but had the density of nuclei? express your answer to two significant figures and include the appropriate units.