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Angular velocity, acceleration, including centripetal and coriolis acceleration

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In order to define angular velocity, consider a particle $P$ moving in a $XOY$ reference plane, as diagramed in Figure 1. Let $x$ be the projection of $P$ on the $OX$ axis, $y$ the projection of $P$ on the $OY$ axis, $r$ the length of $OP$ , and $\theta$ the $\angle{POX}$ angle.

We can then write that:

$x = r\cos\theta$

(5)

$y = r\sin\theta$

(6)

By differentiating these equations with respect to time, we have:

$\dot x = \frac{dx}{dt} = \dot r \cos\theta - r\dot \theta \sin\theta$

(7)

$\dot y = \frac{dy}{dt} = \dot r \sin\theta + r\dot \theta \cos\theta$

(8)

We can write the tangential component of velocity, $v_T$ , which is the component of velocity perpendicular to $OP$ , as:

$v_T = \dot y \cos\theta - \dot x \sin\theta$

(9)

and, by using the expressions of $\dot x$ and $\dot y$ from equations (#3) and (#4) respectively, we obtain:

$v_T = r \dot \theta$

(10)

We define the angular velocity $\omega$ as the rate of change of $\theta$ with respect to time:

$\omega = \frac{d\theta}{dt}$

(11)

Using (#7) in (#6) we get that:

$v_T = r\omega$

(12)

from which the angular velocity becomes:

$\omega = \frac{v_T}{r}$

(13)

In order to define the acceleration, we first have to calculate $\ddot x$ and $\ddot y$ . To do this we differentiate equations (#3) and (#4) with respect to time, when we obtain that:

$\ddot x = \ddot r \cos\theta - \dot r \sin\theta - r\ddot \theta^2 \cos\theta - r \ddot \theta \sin\theta$

(14)

$\ddot y = \ddot r \sin\theta + \dot r \dot \theta \cos\theta + r \dot \theta^2 \sin\theta +r \ddot \theta \cos\theta$

(15)

We can write the radial component of acceleration, $a_r$ , which is the component of acceleration in the direction of $OP$ , as:

$a_r = \ddot x \cos\theta + \ddot y \sin\theta$

(16)

and, by using the expressions of $\ddot x$ and $\ddot y$ from equations (#10) and (#11) respectively, we get:

$a_r = \ddot r - r \dot \theta^2$

(17)

We can also write (#13), by using (#7), as:

$a_r = \dot v - r\omega^2$

(18)

where $\dot v$ is the rate of change of velocity, while the $r\omega^2$ term is called the centripetal acceleration ( $a_c$ ).

The tangential component of acceleration on the other hand, $a_T$ , can be written as:

$a_T = \ddot y \cos\theta - \ddot x \sin\theta$

(19)

and, again by replacing $\ddot x$ and $\ddot y$ from equations (#10) and (#11) respectively, we obtain:

$a_T = r \ddot \theta + 2 \dot r \dot \theta$

(20)

We define the angular acceleration $\alpha$ as the rate of change of the angular velocity $\omega$ with respect to time:

$\alpha = \frac{d\omega}{dt} = \ddot \theta$

(21)

Equation (#16) can also be written, by taking into account (#17), as:

$a_T = r \alpha + 2v\omega$

(22)

where the $2v\omega$ term is called the compound supplementary acceleration, or the Coriolis component of acceleration ( $a_C$ ).

Example 1 [imperial]

Problem

Consider that the hard disk of a computer is circular and rotates with an angular velocity of 7200 revolutions per minute. Calculate the linear velocity (in miles per hour) of a particle which is found 2 inches away from the center of the hard disk.

Workings

The angular velocity of the hard disk is: