= Centripetal force
{title2=$F_r=-mv^2/r$}
A <centripetal force> is the inward resultant <force> needed for circular motion. <Newton's second law> and <centripetal acceleration> give magnitude $mv^2/r$ for a particle of <mass> $m$, speed $v$, and path radius $r$. It is a role played by an actual interaction, such as <tension> or an <electric force>, rather than an additional interaction to add to a <free-body diagram>. A purely radial <centripetal force> is perpendicular to the <velocity>, so it does no <work> and leaves the speed unchanged.
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