When a body moves with some friction on a surface,

Two wedges each of mass $600 \ g$ are placed next to each other on a rough horizontal surface. The coefficient of static friction between the wedges and the surface is $0.4$. $A$ cube of mass $M$ is balanced on the wedges as shown in the figure. If there is no friction between the cube and wedges,the largest mass $M$ of the cube that can be balanced without motion of the wedges is . . . . . . $kg$.

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 $k = \frac{mg}{a}$. If $P$ and $Q$ are in equilibrium when $PA = AQ = 3a$,then the least coefficient of friction between the ring and the wire is $\mu$. Find the value of $\mu + \sqrt{3}$.

$A$ particle of mass $m$ falls from rest through a resistive medium having a resistive force, $F = -kv$, where $v$ is the velocity of the particle and $k$ is a constant. Which of the following graphs represents velocity $(v)$ versus time $(t)$?

What is the ratio of the accelerations of both systems?

$A$ metal block is resting on a rough wooden surface. $A$ horizontal force applied to the block is increased uniformly with time,i.e.,$F = kt$. Which of the following curves correctly represents the velocity $v$ of the block as a function of time $t$?