JEE Advanced Model Paper 13 with solutions for online practice

A spring is attached with a block of mass m and a fixed horizontal rod. The blockis lying on a smooth horizontal table and initially the spring is vertical andunstretched. Natural length of spring is 3l0 . A constant horizontal force F isapplied on the block so that block moves in the direction of force. When length ofthe spring 5l0 , block is about to leave the contact with the table. If initialand final velocity of block is zero, F =

mg

4

+

mg

N

. What is 'N'?