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The angular velocity of a stone whirled in a horizontal circle of radius $r$ at angle $\theta$ is:
A$\omega=\sqrt{\dfrac{g\tan\theta}{r}}$
B$\omega=\dfrac{r\tan\theta}{g}$
C$\omega=\sqrt{\dfrac{r\tan\theta}{g}}$
D$\omega=\dfrac{g\tan\theta}{r}$
Answer & Solution
Correct answer: A. $\omega=\sqrt{\dfrac{g\tan\theta}{r}}$
1. From the two force components, $v^2=rg\tan\theta$.
2. For circular motion $v=\omega r$, so $v^2=\omega^2r^2$.
3. Equating: $\omega^2r^2=rg\tan\theta$, giving $\omega^2=\dfrac{g\tan\theta}{r}$.
4. Taking the square root gives $\omega=\sqrt{\dfrac{g\tan\theta}{r}}$.
5. The revolutions per second follow as $\dfrac{\omega}{2\pi}$. Omitting the square root, a recorded error, leaves the wrong units entirely.
_Source: NECTA ACSEE 2023 Physics 131/1, Question 3: Mechanics (Uniform Circular Motion)_