A bob of mass `m` suspended by a light inextensible string of length 'l' from a fixed point. The bob is given a speed of `sqrt(6gl)`. Find the tension in the string when string deflects through an angle `120^@` from the vertical.

Explanation

By COE theorem,
`u=sqrt(6gl)`
`1/2mu^2=mg(1-cos120)+1/2mv^2`
gives `v=sqrt(3gl)`
At point B, `T+mg cos 60^@=(mv^2)/(l)`
But putting `v=sqrt(3gl)`, we get,
`T=5/2mg`