The least coefficient of friction letting a car take an unbanked curve of radius $r$ at speed $v$ is:

1. On an unbanked curve, friction alone supplies the centripetal force. 2. So $\mu mg=\dfrac{mv^2}{r}$, since the maximum friction available is $\mu mg$. 3. The mass cancels from both sides. 4. Rearranging gives $\mu=\dfrac{v^2}{rg}$. 5. Dropping the $g$ leaves $\dfrac{v^2}{r}$, which is an acceleration and not dimensionless; inverting gives $\dfrac{gr}{v^2}$. Both were recorded errors. _Source: NECTA ACSEE 2023 Physics 131/1, Question 3: Mechanics (Uniform Circular Motion)_