Figure shows two block system, $4 \,kg$ block rests on a smooth horizontal surface, upper surface of $4 \,kg$ is rough. A block of mass $2 \,kg$ is placed on its upper surface. The acceleration of upper block with respect to earth when $4 \,kg$ mass is pulled by a force of $30 \,N$, is ......... $m / s ^2$
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(b)
It both move together
$a=\frac{30}{(4+2)}=5 m / s ^2$
$2 \,kg$ will move due to frictional force
$F=m a \Rightarrow f=2(5)=10 N$
and limiting friction $f_L=(0.8)(2 g )=16 \,N$
$\Rightarrow$ Friction is sufficient to move both block together hence $a=5 \,m / s ^2$
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