Two bodies of masses m1 and m2 are initially at rest placed infinite distance apart. They are then allowed to move towards each other under mutual gravitational attraction. Their relative velocity when they are r distance apart is
From conservation of mechanical energy : decrease in gravitational potential energy = increase in kinetic energy.
or Gm1m2r=12μv2r
Here, μ=m1m2m1+m2 = reduced mass
and vr = relative velocity of approach
∴ vr=√2G(m1+m2)r