# Please explain equation ii and iii and f12=-f21

${f}_{21}istheforceexertedby{q}_{1}on{q}_{2}\phantom{\rule{0ex}{0ex}}{f}_{12}istheforceexertedby{q}_{2}on{q}_{1}\phantom{\rule{0ex}{0ex}}Theseforcesareequalinmagnitudebutwillhaveoppositedirection$

Thus equation ii and iii are vectors with same magnitude but with opposite direction.

For vectors having opposite direction negative sign is used.

so ${f}_{21}=-{f}_{12}$

Regards

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