question_answer

A block A with mass 100 kg is resting on another block B of mass 200 kg.  As shown in figure a horizontal rope tied to a wall holds it.  The coefficient of friction between A and B is 0.2 while coefficient of friction between B and the ground is 0.3. The minimum required force F to start moving B will be     [RPET 1999]

A)             900 N

B)               100 N

C)             1100 N

D)             1200 N

Correct Answer: C

Solution :

                  \[F={{f}_{AB}}+{{f}_{BG}}\]             \[={{\mu }_{AB}}{{m}_{a}}g+{{\mu }_{BG}}({{m}_{A}}+{{m}_{B}})g\]             \[=0.2\times 100\times 10\]                         \[+0.3(300)\times 10\]             \[=200+900=1100\ N\]