First slide
Work done by Diff type of forces

Two plates whose masses are m1 and m2  respectively are connected by a massless spring as shown in figure.  What minimum force should be applied to the upper plate for it to lift the lower one after the force is removed.


The lower plate will rise if spring force on it greater than its weight  Kx2>m2g 
suppose it is compressed by x1;12Kx12=12Kx22+m1gx1+x2
Hence x1>2m1Kg+m2gK F+m1g=Kx1F>m1+m2g

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