[" Two particles of equal mass go round a "],[" circle of radius "R" under the action of their "],[" mutual gravitational attraction.It speed of "],[" each particle is."],[[" (1) "v=(1)/(2R)sqrt((1)/(Gm))," (2) "v=sqrt((4Gm)/(2R))],[" (3) "v=(1)/(2)sqrt((Gm)/(R))," (4) "v=sqrt((4Gm)/(R))]]