9th Class Science Force and laws of motion Question Bank Force and Laws of Motion

  • question_answer
    Pushing force making an angle \[\theta \]to the horizontal is applied on a block of weight W placed on a horizontal table. If the angle of friction is\[\phi \] , the magnitude of force required to move the body is equal to

    A)  \[\frac{W\,\cos \phi }{\cos \,(\theta -\phi )}\]  

    B)         \[\frac{W\,\sin \phi }{\cos \,(\theta -\phi )}\]

    C)  \[\frac{W\,\tan \phi }{\sin (\theta -\phi )}\]       

    D)         \[\frac{W\,\sin \phi }{g\sin (\theta -\phi )}\]

    E)        None of these  

    Correct Answer: E

    Solution :

                \[R=(W+P\sin \theta )\] Now      \[{{F}_{f}}=\mu R=\mu (W+P\sin \theta )\] But       \[\mu =\tan \phi \] \[\therefore \]      \[{{F}_{f}}=\tan \phi (W+P\sin \theta )\] Also,     \[{{F}_{f}}=P\cos \theta \] Eliminating\[P\], we have             \[{{F}_{f}}=\frac{W\sin \phi }{\cos (\theta +\phi )}\] So, none of the options is correct.


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