Rotational Kinematics and Dynamics

Angular Quantities

Angular position: $\theta$ (radians)

Angular velocity: $\omega = \frac{d\theta}{dt}$

Angular acceleration: $\alpha = \frac{d\omega}{dt} = \frac{d^2\theta}{dt^2}$

Relationship to Linear Quantities

For point at distance $r$ from axis:

Arc length: $s = r\theta$

Linear velocity: $v = r\omega$

Tangential acceleration: $a_t = r\alpha$

Centripetal acceleration: $a_c = \frac{v^2}{r} = r\omega^2$

Rotational Kinematics

For constant angular acceleration $\alpha$:

$\omega = \omega_0 + \alpha t$

$\theta = \theta_0 + \omega_0 t + \frac{1}{2}\alpha t^2$

$\omega^2 = \omega_0^2 + 2\alpha(\theta - \theta_0)$

(Analogous to linear kinematics)

Variable Angular Acceleration

When $\alpha(t)$ is given:

$\omega(t) = \omega_0 + \int_0^t \alpha(t') \, dt'$

$\theta(t) = \theta_0 + \int_0^t \omega(t') \, dt'$

When $\alpha(\theta)$ is given, use: $\alpha = \frac{d\omega}{dt} = \frac{d\omega}{d\theta}\frac{d\theta}{dt} = \omega\frac{d\omega}{d\theta}$

Torque

Definition: $\vec{\tau} = \vec{r} \times \vec{F}$

Magnitude: $\tau = rF\sin\phi$

where $\phi$ is angle between $\vec{r}$ and $\vec{F}$.

Perpendicular distance form: $\tau = r_{\perp}F = rF_{\perp}$

Rotational Dynamics

Newton's second law for rotation: $\tau_{net} = I\alpha$

where $I$ is moment of inertia.

Differential form: $\tau = I\frac{d\omega}{dt}$

Work and Power in Rotation

Work by torque: $W = \int_{\theta_1}^{\theta_2} \tau \, d\theta$

For constant torque: $W = \tau\Delta\theta$

Rotational power: $P = \tau\omega$

(Analogous to $P = Fv$ for linear motion)

Rotational Kinetic Energy

$KE_{rot} = \frac{1}{2}I\omega^2$

Work-energy theorem: $W_{net} = \Delta KE_{rot} = \frac{1}{2}I\omega_f^2 - \frac{1}{2}I\omega_i^2$

Combined Translation and Rotation

For rolling object:

Total kinetic energy: $KE = KE_{trans} + KE_{rot} = \frac{1}{2}Mv_{cm}^2 + \frac{1}{2}I_{cm}\omega^2$

Rolling without slipping: $v_{cm} = R\omega$

where $R$ is radius.

Example: Rolling Down Incline

Energy conservation: $Mgh = \frac{1}{2}Mv^2 + \frac{1}{2}I\omega^2$

Using $v = R\omega$ and $I = \beta MR^2$:

$Mgh = \frac{1}{2}Mv^2 + \frac{1}{2}\beta MR^2\frac{v^2}{R^2}$

$gh = \frac{1}{2}v^2(1 + \beta)$

$v = \sqrt{\frac{2gh}{1 + \beta}}$

where $\beta$ depends on shape:

• Solid cylinder: $\beta = 1/2$
• Solid sphere: $\beta = 2/5$
• Hoop: $\beta = 1$

Angular Impulse

$\int_{t_1}^{t_2} \tau \, dt = \Delta L = I\Delta\omega$

where $L = I\omega$ is angular momentum.