A girl of mass `40kg` sits on a swing formed by a rope `6m` long. A person pulls the swing to a side so that the rope makes an angle of `60^(@)` with vertical . What is the gain in `P.E.` of the girl.

Explanation

In figure, `OA=OB=6m`
In `DeltaOBA, ` `cos60^(@)=(OC)/(OB)`
or `OC=OB` `cos60^(@)=6xx(1)/(2)=3m`
`:.` Height through which girl rises,
`h=OA-OC=6-3=3m.`
Potentia energy gained by girl
`=mgh=40xx9.8xx3=1176J`