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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Bidyut sinha
As for simple pendulum given r=l, that is the the length of the pendulum and and it is CONSTANT.Because for simple pendulum length is not varying..It remains constant.. so r= constant ,so rdot means derivative of r with respect to t =0 .. because derivative of constant is 0. and Pr is canonical momentum = del L/del rdot... means partial derivative of L with respect to rdot...Now as rdot is 0.so Pr is also 0.