In the diagram, $BAC$ is a rigid fixed rough wire and angle $BAC$ is $60^o$. $P$ and $Q$ are two identical rings of mass $m$ connected by a light elastic string of natural length $2a$ and elastic constant $\frac{mg}{a}$. If $P$ and $Q$ are in equilibrium when $PA = AQ = 3a$ then the least coefficient of friction between the ring and the wire is $\mu$. Then value of $\mu + \sqrt 3 $ is :-

A uniform chain of $6\, m$ length is placed on a table such that a part of its length is hanging over the edge of the table. The system is at rest. The co-efficient of static friction between the chain and the surface of the table is $0.5$, the maximum length of the chain hanging from the table is.......$m.$

A $\vec F\,\, = \,\,\hat i\, + \,4\hat j\,$ acts on block shown. The force of friction acting on the block is :

The limiting value of static friction between two contact surfaces is ........... 

Which one of the following statements is incorrect? 

Which one of the following is not used to reduce friction