question_answer

A block A weighing 100 kg rests on a block B and is tied with a horizontal string to the wall   at C. Block B weighs 200kg. The coefficient of friction between A and B is 0.25 and between B and the surface is \[\frac{1}{3}.\] The horizontal force P necessary to move the block B should be \[(g=10m/{{s}^{2}})\]

A)  1150 N         

B)  1250 N

C)  1300 N         

D)  1420 N

Correct Answer: B

Solution :

 Friction between block A and block B and between block B and surface will oppose the P. \[\therefore \] \[P={{F}_{AB}}+{{F}_{BS}}\] \[={{\mu }_{AB}}{{m}_{A}}g+{{\mu }_{BS}}({{m}_{A}}+{{m}_{B}})g\] \[=0.25\times 100\times 10+\frac{1}{3}(100+200)\times 100\] \[=1250\,N\]