see u really need not get the vector law in the picture and confuse ur self . OK then centripetal force is nothig but any external force like gravitation,tension,,friction etc. The concept of centripetal force is applied to be negative as per ur question above when the magnitude is mv^2/r , but in case of non uniform circular motion the centripetal force is negative in magnitude . explanation of neg force::::::::::
if speed of a particle in a circle is not constant then the acc has both radial and tangential acceleration . So when the accn is radially inward that is towards the centre then then it is negative incase of a car moving in circular path and then drifting in the direction opposite to the direction of the centripetal force . I hope that this gave u some idea of the cases of when centripetal is negative and when is it positive.
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