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Given in the figure are two blocks A and B of weight 20 N and 100 N, respectively. These are being pressed against a wall by a force F as shown.  If the coefficient of friction between the blocks is 0.1 and between block B and the wall is 0.15, the frictional force applied by the wall on block B is :

  • Option 1)

    100 N

  • Option 2)

    80 N

  • Option 3)

    120 N

  • Option 4)

    150 N


Answers (1)

As we learnt in

Motion of Block in Contact -

- wherein


a=\frac{F_{net}}{m_{1}+m_{2} }



Static Friction -

Self adjusting force because it changes itself according with the applied force.

- wherein

It always equal to net external force. Static friction (F = P) using diag.



N = F

fr1 = 20 N

fr2 = 100 + 20 = 120 N

Where assume two blocks as one system due to equilibrium. 

fr =120 N

Correct option is 3.



Option 1)

100 N

This is an incorrect option.

Option 2)

80 N

This is an incorrect option.

Option 3)

120 N

This is the correct option.

Option 4)

150 N

This is an incorrect option.

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