$A$ chain is lying on an absolutely smooth table,with half of it hanging over the edge of the table as shown in figure $(a)$. The time it takes to slip off the table is affected if two equal weights are attached,one to each end as shown in figure $(b)$.
$t_a$ : time taken to slip in situation $(a)$
$t_b$ : time taken to slip in situation $(b)$

Two spheres $P$ and $Q$ of equal masses $m$ are attached to a string of total length $2\, m$ as shown in the figure. The string and the spheres are then whirled in a horizontal circle about $O$ at a constant angular velocity $\omega$. The distance $OP = 1\, m$ and $PQ = 1\, m$. What is the value of the ratio $\left( \frac{\text{Tension in the string between } P \text{ and } Q}{\text{Tension in the string between } O \text{ and } P} \right)?$

The figure shows a uniform rod of length $30 \,cm$ having a mass of $3.0 \,kg$. The rod is pulled by constant forces of $20 \,N$ and $32 \,N$ as shown. Find the force exerted by the $20 \,cm$ part of the rod on the $10 \,cm$ part (all surfaces are smooth).

Two spheres $P$ and $Q$,each of mass $200 \text{ g}$,are attached to a string of length $1 \text{ m}$ as shown in the figure. The string and the spheres are then whirled in a horizontal circle about $O$ at a constant angular speed. The ratio of the tension in the string between $P$ and $Q$ to that between $P$ and $O$ is (where $P$ is at the mid-point of the line joining $O$ and $Q$).

The figure shows a boy on a horizontal platform $A$ on a smooth horizontal surface,holding a rope attached to a box $B$. The boy pulls the rope with a constant force of $50 \ N$. (The boy does not slip over the platform). The combined mass of platform $A$ and the boy is $250 \ kg$,and that of box $B$ is $500 \ kg$. The velocity of $A$ relative to the box $B$,$5 \ s$ after the boy on $A$ begins to pull the rope,will be ............ $m/s$.

$A$ balloon of mass $M$ descends with an acceleration $a (< g)$. What mass needs to be removed from the balloon so that it starts ascending with acceleration $a$?