J & K CET Engineering J and K - CET Engineering Solved Paper-2010

  • question_answer
    Three forces \[{{F}_{1}},{{F}_{2}}\] and \[{{F}_{3}}\] together keep a body in equilibrium. If \[{{F}_{1}}=3N\]along the positive x-axis, \[{{F}_{2}}=4N\] along the positive y-axis, then the third force \[{{F}_{3}}\] is

    A) \[5\text{ }N\]making an angle \[\theta ={{\tan }^{-1}}\left( \frac{3}{4} \right)\] with the negative y-axis

    B) \[5\text{ }N\]making an angle \[\theta ={{\tan }^{-1}}\left( \frac{4}{3} \right)\] with the negative y-axis

    C) \[7\text{ }N\]making an angle \[\theta ={{\tan }^{-1}}\left( \frac{3}{4} \right)\] with the negative y-axis

    D)  \[7\text{ }N\] making an angle \[\theta ={{\tan }^{-1}}\left( \frac{4}{3} \right)\] with the negative y-axis

    Correct Answer: A

    Solution :

    Resultant,   \[R=\sqrt{{{x}^{2}}+{{y}^{2}}}\] \[R=\sqrt{{{3}^{2}}+{{4}^{2}}}\] \[R=\sqrt{25}=5N\] and \[\cos \theta =\frac{x}{R}=\frac{3}{5}\] \[sin\theta =\frac{y}{R}=\frac{4}{5}\] then \[\tan \theta =\frac{y}{x}=\frac{4}{3}\] \[\theta ={{\tan }^{-1}}\left( \frac{4}{3} \right)\] with negative y-axis.


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