A pendulum bob of mass 0.5kg hanging from a vertical string of length 20m attached to a fixed point is pulled to one side with the string taut until it makes an angle 30° to the vertical. Calculate the work done on the bob?

1 Answer
Mar 16, 2018

Using Law of conservation of energy and assuming that pendulum is ideal and no energy is lost in friction,

Work done on the bob#=# Increase in potential Energy of bob

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Increase in potential Energy of bob#=mgDeltah#
where #m# is mass of bob, #g=9.81\ ms^-2# is acceleration due to gravity and #Deltah# is change in height of bob.

As shown in the figure, #Deltah=L-Lcostheta#. Inserting given values we get

Work done on the bob#=0.5xx9.81(20-20\ cos30^@)#
#=>#Work done on the bob#=0.5xx9.81xx20(1- sqrt3/2)=13.1\ J#