Q.

Two bodies, each of mass M, are kept fixed with a separation 2L. A particle of mass m is projected from the midpoint of the line joining their centres, perpendicular to the line. The gravitational constant is G. The correct statement(s) is (are)                      [2013]

1 The minimum initial velocity of the mass m to escape the gravitational field of the two bodies is 4GML  
2 The minimum initial velocity of the mass m to escape the gravitational field of the two bodies is 2GML  
3 The minimum initial velocity of the mass m to escape the gravitational field of the two bodies is 2GML  
4 The energy of the mass m remains constant  

Ans.

(2, 4)

From conservation of mechanical energy,

-GMmL-GMmL+12mv2=0+0

or, 12mv2=2GMmL        v=4GML=2GML

Total energy of mass 'm' is conserved as there is no external force involved.