Two objects of mass m and 4 m are at rest at an infinite separation. They move towards each other under mutual attraction. If G is the universal gravitational constant. Then at separation r.

Initially potential and kinetic both energies zero and from conservation of   mechanical energy total energy of the two object zeroFurther, decrease in P.E = increase in K.E. Gm4mr=12μvr2 vr=10GmrTotal K.E. =  Gm4mr=4Gm2r