Answer:
As no external force is applied
on the system,
According to work energy principle, total work
done = change In K.E. = final K.E. ? initial K.E.
\[\therefore \] \[{{W}_{ext}}+{{W}_{\operatorname{int}}}=0-\left(
\frac{1}{2}m{{\upsilon }^{2}}+\frac{1}{2}m{{\upsilon }^{2}}
\right)=-m{{\upsilon }^{2}}\] or \[0+{{W}_{\operatorname{int}}}=-5{{\left(
2 \right)}^{2}}=-20J\], i.e., \[{{W}_{\operatorname{int}}}=-20J\]
Negative sign implies that internal force of action and reaction
act on the two blocks in a direction opposite to their motion.
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