A positive lens has a focal length of 15 cm. An object is located at a distance of 3 cm from the lens. Determine, in cm, the location of the image.

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?