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Definition Of Angular Momentum For A Particle

L=r×p=r×mv\vec{L} = \vec{r} \times \vec{p} = \vec{r} \times m\vec{v}

Where:

  • r\vec{r} = position from reference point
  • p\vec{p} = linear momentum

Units: kg - m2/s

L=r×pL = \mathbf{r} \times \mathbf{p} (Particle)

Vector cross product form.

L = Iω (rigid body about fixed axis):

Simplified angular momentum.

L=IωL = I\omega

For extended rigid body rotating about fixed axis.

Total Angular Momentum: L_total = Sigma (r_i × p_i) or L_total = I_total ω:

Sum of angular momenta.

Discrete system: Ltotal=iri×pi\vec{L}_{total} = \sum_{i} \vec{r}_i \times \vec{p}_i

Rigid body: L=IωL = I\omega

Total Angular Momentum: Ltotal=Σ(ri×pi)L_{total} = \Sigma (\mathbf{r}_i \times \mathbf{p}_i) Or Ltotal=ItotalωL_{total} = I_{total} \omega (For Rigid Bodies)

Total Angular Momentum: L_total = Sigma (r_i × p_i) or L_total = I_total ω:

Sum of angular momenta.

Discrete system: Ltotal=iri×pi\vec{L}_{total} = \sum_{i} \vec{r}_i \times \vec{p}_i

Rigid body: L=IωL = I\omega

Linitial=LfinalL_{initial} = L_{final} (If Στext=0\Sigma\tau_{ext} = 0)

Li=Lf\vec{L}_i = \vec{L}_f

Applications:

  • Ice skater spinning (arms in/out -> changes I, ω changes to conserve L)
  • Planetary orbits (Kepler's second law)
  • Collisions with rotation

Integral Form: W=τdθW = \int \tau d\theta

Work by torque through angular displacement.

W=θiθfτ(θ)dθW = \int_{\theta_i}^{\theta_f} \tau(\theta) \, d\theta

Work-energy Theorem For Rotation: Wnet=ΔKrot=12Iωf212Iωi2W_{net} = \Delta K_{rot} = \frac{1}{2}I \omega_f^2 - \frac{1}{2}I \omega_i^2

Work-Energy Theorem for Rotation: W_net = DeltaK_rot = (1/2)Iω_f2 - (1/2)Iω_i2:

Net work equals change in rotational kinetic energy.

Wnet=ΔKrot=Krot,fKrot,iW_{net} = \Delta K_{rot} = K_{rot,f} - K_{rot,i}

Wnet=12Iωf212Iωi2W_{net} = \frac{1}{2}I\omega_f^2 - \frac{1}{2}I\omega_i^2

Instantaneous Power: P=dWdt=τωP = \frac{dW}{dt} = \tau \omega

P=τωP = \tau \omega

Relationship to Linear Power: Analogous to P = F - v in Linear Motion:

Rotational power analogous to translational.

  • τF\tau \leftrightarrow F
  • ωv\omega \leftrightarrow v

Total Kinetic Energy: 12Mvcm2+12Icmω2\frac{1}{2}Mv_{cm}^2 + \frac{1}{2}I_{cm} \omega^2

Total Kinetic Energy: K_total = (1/2)Mv_cm2 + (1/2)I_cm ω2:

Ktotal=Ktrans+Krot=12Mvcm2+12Icmω2K_{total} = K_{trans} + K_{rot} = \frac{1}{2}Mv_{cm}^2 + \frac{1}{2}I_{cm}\omega^2

Using rolling constraint: Ktotal=12Mvcm2+12Icm(vcmR)2K_{total} = \frac{1}{2}Mv_{cm}^2 + \frac{1}{2}I_{cm}\left(\frac{v_{cm}}{R}\right)^2

Conservation Of Energy (Ktrans+Krot+UK_{trans} + K_{rot} + U)

Complete energy conservation.

Etotal=Ktrans+Krot+U=constantE_{total} = K_{trans} + K_{rot} + U = \text{constant}

12Mvcm2+12Icmω2+U=constant\frac{1}{2}Mv_{cm}^2 + \frac{1}{2}I_{cm}\omega^2 + U = \text{constant}

Conservation Of Linear And Angular Momentum

Both can be conserved (different conditions).

  • Linear momentum: Fnet,ext=0\vec{F}_{net,ext} = 0
  • Angular momentum: τnet,ext=0\vec{\tau}_{net,ext} = 0

Example: collision with rotation conserves both if no external forces/torques.