Question

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

• A. $6\, m / s ^{2}$
• B. $5\, m / s ^{2}$
• C. $8\, m / s ^{2}$
• D. $2\, m / s ^{2}$

Answer 1

Answer: (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 $a=5\, m / s ^{2}$