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☰ Chapter 9: Angular Kinetics

Objectives:

Linear and Angular Quantities


Newton's First Law of Motion (relating to angular motion)

Angular Inertia (Moment of inertia): The property of an object to resist changes in angular motion

Angular inertia depends upon mass and mass distribution

Angular inertia
\(I_b = mk^2 + mr^2 \)
\(I = mk^2 \)

Angular inertia in a runners leg

What is the benefit to having limbs that are tapered (The proximal ends are larger than the distal ends)?



Moment of inertia in a bicycle wheel
cycling


Ice-skating examples of manipulating inertia (Calculation)
Diving examples of manipulating inertia

Angular Momentum: \(H = {I ω}\)

Angular momentum in a non-rigid body can be estimated by:
\(H = Σ({I ω})\)

Angular momentum is conserved when no external torques are created on the body.

Angular momentum conservation requires H to be constant.
However, I and ω can change.

Thought Questions

1. Angular momentum is conserved in which of the following activities:

2. How does "choking up" on a baseball bat affect performance?

3. Does a golf club have greater angular inertia when it is held vertically or horizontally?



Newton's Second Law of Motion (relating to angular motion)

Linear
Angular
\(F=ma\)
\(T=Iα\)

F=ma led to \(FΔt = mΔv\)

In angular motion, \(T=Iα\) leads to \(TΔt = Δ(Iω)\)

A change in angular momentum may result in:
True, false, or "it depends": Angular acceleration of an object indicates the presence of a net external torque.

Newton's Third Law of Motion (relating to angular motion)

For every torque exerted by one object on another, the other object exerts an equal torque back on the first body but in the opposite direction

Long jumping (video)

Frolich article