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Question

A rough vertical board has an acceleration ‘a’ so that a 2 kg block pressing against it does not fall. The coefficient of friction between the block and the board should be

  1. greater than g rightwards arrow over short leftwards arrow divided by rightwards arrow over short leftwards arrow a    
  2. less than g rightwards arrow over short leftwards arrow divided by rightwards arrow over short leftwards arrow a    
  3. equals g rightwards arrow over short leftwards arrow divided by rightwards arrow over short leftwards arrow a    
  4. greater than a rightwards arrow over short leftwards arrow divided by rightwards arrow over short leftwards arrow g    

The correct answer is: greater than g rightwards arrow over short leftwards arrow divided by rightwards arrow over short leftwards arrow a


    For the limiting condition upward friction force between board and block will balance the weight of the block.
    i.e. F greater than m g
    Þ mu left parenthesis R right parenthesis greater than m g
    Þ mu left parenthesis m a right parenthesis greater than m g
    Þ mu greater than fraction numerator g over denominator a end fraction

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