What is spring constant ? On which the work done by a spring depends ?

A container of mass $m$ is pulled by a constant force in which a second block of same mass $m$ is placed connected to the wall by a mass-less spring of constant $k$. Initially the spring is in its natural length. Velocity of the container at the instant compression in spring is maximum for the first time :-
 

A spring of spring constant $ 5 \times 10^3$ $ N/m$  is stretched initially by $5\,cm$ from the unstretched position. Then the work required to stretch it further by another $5\,cm$ is .............. $\mathrm{N-m}$

A block is attached to a spring as shown and very-very gradually lowered so that finally  spring expands by $"d"$. If same block is attached to spring & released suddenly then maximum expansion in spring will be-

Two masses $A$ and $B$ of mass $M$ and $2M$ respectively are connected by a compressed ideal spring. The system is placed on $a$ horizontal frictionless table and given $a$ velocity $u\, \hat k$ in the $z$ -direction as shown in the figure. The spring is then released. In the subsequent motion the line from $B$ to $A$ always points along the $\hat i$ unit vector. At some instant of $\rho$ time mass $B$ has $a$ $x$ -component of velocity as $V_x\, \hat i$ . The velocity ${\vec V_A}$ of as $A$ at that instant is

A smooth semicircular tube $AB$ of radius $R$ is fixed in a verticle plane and contain a heavy flexible chain of length $\pi R$ . Find the velocity $v$ with which it will emerge from the open end $'B'$ of' tube, when slightly displaced