Two disks of masses `m_(1)` and `m_(2)` are connected by a spring of force canstant k. The lower disk of mass `m_(2)`lies on a table and the upper disk is vertically above it. What vertical force F should be applied to the upper disk so that when the force is withdraw, the lower disk is lifted off the table?

Explanation

The situation is shown in fig. Let the applied force F compresses the spring by x, so `F_(s)=kx`. After removal of force, the spring recovers its compressed length and further extends by x. At this instant the force exerted by spring is along upward direction. Thus, to lift off the lower disk, normal reaction N becomes zero.
When force is applied and spring is compressed,
`F=F_(S)+m_(1)g` ...(i)
When `m_(2)` leaves contact (we have, N=0) with ground and spring is stretched,
`F_(S)=m_(2)g` ..(ii)
From Eqs (i) and (ii) we have `F_(min)=(m_(1)+m_(2))g`
So to lift off the lower disk, `Fge(m_(1)+m_(2))g`.