Answer:
Water is a polar molecule
Explanation:
The general solubility rule is that like dissolves like. In short, polar solutes would dissolve in polar solvents. Non polar solutes also dissolves on non polar solvents.
NaCl is an ionic molecule which forms as a result of transfer of electrons from one atom to another. The transfer of electrons makes the atoms oppositely a charged ions and they become polar.
Water is made up of Hydrogen H⁺ ions and hydroxy ions OH⁻. Water is an heteronuclear covalent molecule. Here, there is an unequal sharing of electrons as one atom pulls the shared electron more closer. This leaves a partial negative charge on the more electronegative atom and a partial positive charge on the other.
This polar nature of water NaCl makes dissolution easy.
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The elastic potential energy of a spring is given by

where k is the spring's constant and x is the displacement with respect to the relaxed position of the spring.
The work done by the spring is the negative of the potential energy difference between the final and initial condition of the spring:

In our problem, initially the spring is uncompressed, so

. Therefore, the work done by the spring when it is compressed until

is

And this value is actually negative, because the box is responsible for the spring's compression, so the work is done by the box.