UN Fisika 2016 Pket 1 Nomor 9

Perhatikan gambar berikut!

9

Jika massa batang PR diabaikan, \tan \alpha =\frac{4}{3} dan satuan dinyatakan dalam SI, momen gaya total dengan poros titik O adalah …

A.    1,6F N.m searah jarum jam D. 1,2F N.m berlawanan arah jarum jam
B.     1,6F N.m berlawanan arah jarum jam E. 1,2F N.m searah jarum jam
C.     1,4F N.m searah jarum jam

Penyelesaian

Untuk mengerjakan soal ini, torka searah perputaran jarum jam bernilai – sedangkan torka berlawanan perputaran jarum jam bernilai +.

\tan \alpha =\frac{4}{3} berarti \sin \alpha =\frac{4}{5}

Total torka sistem dengan poros titik O adalah

\text{ }\!\!\Sigma\!\!\text{ }\tau ={{\tau }_{P}}+{{\tau }_{Q}}+{{\tau }_{R}}

\text{ }\!\!\Sigma\!\!\text{  }\!\!\tau\!\!\text{ }=-\left( F\times PO \right)+\left( 2F\times QO \right)-\left( F\sin \alpha \times RO \right)

\text{ }\!\!\Sigma\!\!\text{  }\!\!\tau\!\!\text{ }=-\left( F\times 2 \right)+\left( 2F\times 1 \right)-\left( F\left( \frac{4}{5} \right)\times 2 \right)

\text{ }\!\!\Sigma\!\!\text{  }\!\!\tau\!\!\text{ }=-\left( 2F \right)+\left( 2F \right)-\left( \frac{8}{5}F \right)

\text{ }\!\!\Sigma\!\!\text{  }\!\!\tau\!\!\text{ }=-1,6\text{F }\!\!~\!\!\text{ N}.\text{m}

\text{ }\!\!\Sigma\!\!\text{  }\!\!\tau\!\!\text{ }=1,6\text{F }\!\!~\!\!\text{ N}.\text{m }\!\!~\!\!\text{ searah }\!\!~\!\!\text{ perputaran }\!\!~\!\!\text{ jarum }\!\!~\!\!\text{ jam}

jawaban A

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