A pendulum of mass 1 kg and length `l=1` m is released from rest at angle `theta = 60^(@)`. The power delivered by all the forces acting on the bob at angle `theta = 30^(@)` will be `(g= 10 m//s^(2))`

Options

A. 13.5 W
B. 20.4 W
C. 24.6 W
D. zero

Answer

A

Explanation

(a) Power due to tension =0
Power due to mg =(mg)(v) cos60^(@)
Here `" " v=sqrt(2gh)`
and `" " h=l(cos 30^(@)-cos 60^(@))=1((sqrt3)/(2)-(1)/(2))`
`:. " " v=sqrt(2xx10xx((sqrt3-1)/(2)))=sqrt(10(sqrt3-1))=2.7 ms^(-1)`
`:. "Power",P=(mgv)/(2)=(1xx10)/(2)xx27=13.5 W`