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

  • question_answer
    A body rests on a rough horizontal plane. A force is applied to the body directed towards the plane at an angle \[\phi \]with the vertical. The body can be moved along the plane.

    A)  Only \[\phi \]is more than the angle of friction  

    B)  Only \[\phi \] is less than the angle of friction

    C)  Only \[\phi \] is equal to the angle of friction

    D)  For all values of \[\phi \]

    Correct Answer: A

    Solution :

     We know that angle of friction is the angle which the resultant of force of friction and the normal reaction makes with the direction of normal reaction, which is vertical for an object placed horizontally on a surface. The motion begins when this resultant angle is just greater than the angle of friction. This situation occurs when the applied force is just greater than the force limiting friction. When a suitable force is applied to a block making an angle with the vertical which is equal to the angle of friction, then its horizontal component is equal to the force of limiting friction. When this angle is just greater than the angle of friction, the horizontal component is greater than the force of limiting friction and thus the motion begins.


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