Two heavy spheres each of mass 100kg and radius 0.10m are placed 1.0m apart on a horizontal table. What is the gravitational force and potential at the mid point of the line joining the centres of the spheres? Is an object placed at that point in equilibrium? if so, is the equilibrium stable or unstable?

Explanation

Gravitational field at the mid-point of the line joining the centres of the two spheres
`= (GM)/((r//2)^(2)) (-hat(r)) + (GM)/((r//2)^(2)) hat(r) = 0`
Gravitational potential at the mid point of the line joining the centres of the two spheres is
`V = (-GM)/(r//2) + ((-GM)/(r//2)) = (-4GM)/(r) = (-4 xx 6.67 xx 10^(-11) xx 100)/(1.0) = 2.7 xx 10^(-8) J//kg`
As the effective force on the body placed at md-point is zero, so the body is in equilibrium. If the body is displaced a little towards either mass body from its equilibrium position, it will not return back to its initial position of equilibirum. Hence, the body is in unstable equilibirum.