What is the difference between rotational velocity & rotational frequency? Their units seem to be the same, and I've read that one is a 'scalar' and the other is a 'vector,' but how do they differ?
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There exists indeed a subtle difference, and are often referred to as angular frequency and angular velocity. Both are $\omega$ and have units $\text{s}^{-1}$.
The difference lies in the fact that angular frequency is the magnitude of angular velocity, and is hence a scalar-, instead of vector quantity. The direction of the vector is perpendicular to the plane of rotation.
The angular velocity is actually a pseudovector, and has units $\text{rad s}^{-1}$, but usually the terms are used interchangeably.
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A vector quantity has a direction and a magnitude. The direction of the rotational velocity vector is along the axis around which a body is rotating. Its magnitude is the rotational frequency which tells you how fast the body is rotating.
