Is $0 $ radians an acute angle? I know that an angle less than $\frac {\pi}{2} $ radians is called acute, but under this definition, is an angle that is $0$ radians also considered acute?
 A: According to mathcaptain, which is a website maintained by the publisher Pearson (which publishes math textbooks), an angle of $0$ is called a "zero" angle, an angle of $\pi$ is called a "straight" angle, and an angle of $2\pi$ is called a "complete" angle.
A: Since you wrote

I know that an angle less than $\frac {\pi}{2} $ radians is called acute

then obviously an angle of zero radians is acute, simply because $0<\frac {\pi}{2}$.
A: No, an acute angle is an angle $\theta$ for which $0<\theta<\dfrac{\pi}{2}$.
A: An angle of measure 0 radians is not acute. If an angle is in degrees, it is only acute if it is between 90 and 0, exclusive. For radians, it is acute if it is between π/2 and 0, exclusive. Because the definition of an acute angle doesn't include 0, 0 radians is not acute.
Edit: Additionally, in Wikipedia, "Euclid defines a plane angle as the inclination to each other, in a plane, of two lines which meet each other, and do not lie straight with respect to each other." If the two rays point in the same direction and have a common vertex, then 0 radians isn't an angle at all.
