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Q.

Two hypothetical planets of masses m1 and m2 are at rest when they are infinite distance apart. Because of the gravitational force they move towards each other along the line joining their centres.

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What is their speed when their separation is ‘d’?
(Speed of m1 is v1 and that of m2 is v2)

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a

v1 = v2 

b

v1=m22Gdm1+m2,v2=m12Gdm1+m2

c

v1=m12Gdm1+m2,v2=m22Gdm1+m2

d

v1=m22Gm1,v2=m22Gm2

answer is B.

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Detailed Solution

We choose reference point, infinity, where total energy of the system is zero.
So,

From conservation of energy,

0= 12m1v12+12m2v22-Gm1m2d

and conservation of linear momentum, 

m1v1+m2v2=0 , we get the values of velocities

v1=m22Gdm1+m2, and , v2=m12Gdm1+m2

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