IIT JEE , AIEEE Forum - Physics , Mechanics

anjali

1} A mass of M kg is suspended by a weightless string. The horizontal force that is required to displace it until the string makes an angle of 45° with the initial vertical direction is

(1) Mg (Ö2 - 1) (2) Mg (Ö2 + 1) (3) Mg Ö2 (4) Mg/Ö2

please answer with full explanation

edison

F = Mg (

2 - 1)

polymath

Let L be length of string.The work done in displacing the mass is given by FLsin 45. This work is stored as gravitational potential energy. The mass rises from a length L to Lcos45. Draw the diagram and u will know that.Hence change in gravitational potential energy is Mg (L- L cos 45). Using work- energy theorem,

FLsin 45 = Mg (L - L cos 45)

F= MG(1-cos45)/sin45

F = Mg (

2 - 1)

Hence 1st choice is correct.

polymath

Hope u understand

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