Question

A pendulum in vertical plane having length of string l and mass of Bob M is kept horizontal now the bob is released from horizontal position the tension in string when it makes an angle of 60 degree with vertical is
1) mg/2. (2) 3/4 mg (3) mg (4)
3/2mg

Answer 1

first of all we have to find velocity when pendulum makes an angle 60° with vertical.

using law of conservation of energy theorem,

initial energy = final energy

mgl = mgl(1 - cos60°) + 1/2 mv² [ see diagram

or, mgl = 0.5mgl + 1/2mv²

or, v = √gl

now, T - mgcos60° = mv²/l [ see free body diagram ]

or, T - mg × 1/2 = mgl/l

or, T = mg + mg/2 = 3mg/2

hence, option (4) is correct choice.