Shown in the diagram is a system of two bodies, a block of mass $m$ and a disc of mass $4\ m$ , held in equilibrium. If the string $3$ is burnt, find the acceleration of the disc. Neglect the masses of the pulleys $P$ and $Q$ . The co-efficient of friction between the block and horizontal surface is $0.5$ and friction between disc and string is zero …….. $m/s^2$

$A$ block of mass $M$ is placed on $a$ horizontal surface and it is tied with an inextensible string to $a$ block of mass, as shown in figure. A block of mass $m_0$ is also placed on $M$ The minimum value of $\mu$ between the block $M$ and $m_0$ (taking horizontal surface frictionless) for which all the three blocks move together, is

In figure, two blocks are separated by a uniform strut attached to each block with frictionless pins. Block $A$ weighs $400\,N$, block $B$ weighs $300\,N$, and the strut $A B$ weigh $200\,N$. If $\mu=0.25$ under $B$, determine the minimum coefficient of friction under $A$ to prevent motion.