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Question Number 55217 by peter frank last updated on 19/Feb/19

Answered by mr W last updated on 19/Feb/19

Commented by mr W last updated on 20/Feb/19

(i) is clear, since thread is weightless.    (ii)  tension in thread=T  2T cos (θ/2)=W  2T sin β+γλ=W  ⇒2T(cos (θ/2)−sin β)=γλ  ⇒T=((γλ)/(2(cos (θ/2)−sin β)))    an other way:  T=γR  R(cos (θ/2)−sin β)=(λ/2)  ⇒R=(λ/(2(cos (θ/2)−sin β)))  ⇒T=((γλ)/(2(cos (θ/2)−sin β)))

(i)isclear,sincethreadisweightless.(ii)tensioninthread=T2Tcosθ2=W2Tsinβ+γλ=W2T(cosθ2sinβ)=γλT=γλ2(cosθ2sinβ)anotherway:T=γRR(cosθ2sinβ)=λ2R=λ2(cosθ2sinβ)T=γλ2(cosθ2sinβ)

Commented by peter frank last updated on 20/Feb/19

why cos (θ/2)?

whycosθ2?

Commented by mr W last updated on 20/Feb/19

you can see in my diagram. if θ is  the angle between the two parts of  the thread at midpoint, then (θ/2) is  the angle between thread and vertical  direction.

youcanseeinmydiagram.ifθistheanglebetweenthetwopartsofthethreadatmidpoint,thenθ2istheanglebetweenthreadandverticaldirection.

Commented by mr W last updated on 20/Feb/19

or you draw a diagram showing which  angles are meant with β and θ in the  question.

oryoudrawadiagramshowingwhichanglesaremeantwithβandθinthequestion.

Commented by peter frank last updated on 22/Feb/19

thank sir

thanksir

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