What is the centrifugal force?

Centrifugal Force

Suppose a block mass m lying on a horizontal, frictionless turntable is connected to a string as in figure. According to an inertial observer, if the block rotates uniformly, it undergoes an acceleration of magnitude  v²/r, where v is its tangenial speed. The inertial observer concludes that this centripetal acceleration is provided by the force exerted by the string T, and writes Newton’s second law T = mv²/r.

According to non – inertial observer attached to the turntable, the block is at rest. Therefore, in applying Newton’s second law, this observer introduces a fictitious outward force of magnitude mv²/r. According to the non – linear observer, this outward force balances the force exerted by the string and therefore T – mv²/r = 0.