The potential energy of a weight less spring compressed by a distance $ a $ is proportional to

A body of mass $1\,kg$ falls freely from a height of $100\,m,$ on a platform of mass $3\,kg$ which is mounted on a spring having spring constant $k = 1.25 \times 10^6\, N/m.$ The body sticks to the platform and the spring’s maximum compression is found to be $x.$ Given that $g = 10\,ms^{-2},$ the value of $x$ will be close to ................ $\mathrm{cm}$

The spring extends by $x$ on loading, then energy stored by the spring is :(if $T$ is the tension in spring and $k$ is spring constant)

$A$ ball of mass $m$ is attached to the lower end of light vertical spring of force constant $k$. The upper end of the spring is fixed. The ball is released from rest with the spring at its normal (unstretched) length, comes to rest again after descending through a distance $x.$

An elastic spring under tension of $3 \mathrm{~N}$ has a lengtha. Its length is $b$ under tension $2 \mathrm{~N}$. For its length$(3 a-2 b)$, the value of tension will be_______. $\mathrm{N}$.

Define spring constant and write its unit.