Two uniform soild spheres of equal radii `R` but mass `M` and `4M` have a centre to centre separation `6 R`, as shows in Fig. (a) The two spheres are held fixed. A projectile of mass` m` is projected from the surface of the sphere of mass `M` directly towards the centre of teh second. Obtain an expression for the minimum speed `upsilon` of the projectile so that it reaches the surface of second sphere.

Options

A. `sqrt((4)/(5)(GM)/(R ))`
B. `sqrt((5)/(4)(GM)/(R ))`
C. `sqrt((3)/(5)(GM)/(R ))`
D. `sqrt((5)/(3)(GM)/(R ))`

Answer

C

Explanation

( c)
Let the projectile of mass m be fired with minimum velocity, v from the surface of sphere of mass M to reach the surface of sphere of mass 4M. Let N be neutral point at a distance r from the centre of the sphere of mass M. At neutral point N,
`(GMm)/(r^(2))=(G(4M)m)/((6 R-r)^(2))`
`(6R-r)^(2)=4r^(2)`
`6R-r=pm2r or r=2R or -6R`
The point r= -6R does not conern us. Thus, ON=r=2R
It is sufficient to project the projectile with a speed which would enable it to reach N. Thereafter, the greater gravitational pull of 4M would suffice.
The mechanical energy at the surface of M is
`E_(i)=(1)/(2)mv^(2)-(GMm)/(R )-(G (4M)m)/(5R)`
AT the neutral point, N the speed approaches zero.
`:.` The mechanical enery at N is
`E_(N)=-(GMm)/(2R)-(G(4M)m)/(4R)= -(GMm)/(2R)-(GMm)/(R )`
According to law of conservation of mechanical energy,
`E_(i)=E_(N)`
`(1P)/(2)mv^(2)-(GMm)/(R )-(4 GMm)/(5R)= - (GMm)/(2 R)-(GMm)/(R )`
`v^(2)f=(2GM)/(R)[(4)/(5)-(1)/(2)]=(3)/(5)(GM)/(R ) or v=((3)/(5)(GM)/(R))^(1//2)`