什么是 Uniform circular motion？

A body is in uniform circular motion when its trajectory is a circle traveled at constant speed. The velocity, tangent to the circle, changes direction at every instant — so it is never constant.

A motion is uniform circular when the trajectory is a circle of radius $R$ and the speed $v$ is constant. The period $T$ is the duration of one lap, and the frequency $f = 1/T$ (in $\mathrm{Hz}$) the number of laps per second: $v = 2\pi R/T = 2\pi R f$.