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.

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(m1 + m2) g

(2m1 + m2) g

(m1 + 2m2) g

2(m1 + m2) g

answer is A.

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Detailed Solution

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+x2Hence x1>2m1Kg+m2gK F+m1g=Kx1F>m1+m2g

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