A spring is attached with a block of mass m and a fixed horizontal roof as shown. The block is lying on a smooth horizontal table and initially the spring is vertical and unstretched. Natural length of spring is `3l_(0)`. A constant horizontal force F is applied on the block so that block moves in the direction of force. When length of the spring becomes `5l_(0)`, block leaves contact with the table. Find the constant force F, if initial and final velocity of block is zero.

Answer

`F=(5)/(12)`mg.