Figure shows two block system, 4 ,kg block rests on a smooth horizontal surface, upper surface of 4

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4-2.Friction

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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 ......... $m / s ^2$

A

$6$

B

$5$

C

$8$

D

$2$

Solution

(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 $a=5 \,m / s ^2$

Standard 11

Physics

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