10: Rotational Motion and Angular Momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important quantity in physics because it is a conserved quantity – the angular momentum of a system remains constant unless acted on by an external torque.
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- 10.0: Prelude to Rotational Motion and Angular Momentum
- Why do tornadoes spin at all? And why do tornados spin so rapidly? The answer is that air masses that produce tornadoes are themselves rotating, and when the radii of the air masses decrease, their rate of rotation increases. An ice skater increases her spin in an exactly analogous manner. The skater starts her rotation with outstretched limbs and increases her spin by pulling them in toward her body. The same physics describes the exhilarating spin of a skater and the wrenching force of a torna
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- 10.1: Angular Acceleration
- Angular velocity is not constant when a skater pulls in her arms, when a child starts up a merry-go-round from rest, or when a computer’s hard disk slows to a halt when switched off. In all these cases, there is an angular acceleration, in which ω changes. The faster the change occurs, the greater the angular acceleration. Angular acceleration α is defined as the rate of change of angular velocity.
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- 10.2: Kinematics of Rotational Motion
- Just by using our intuition, we can begin to see how rotational quantities like θ,ω and α are related to one another. For example, if a motorcycle wheel has a large angular acceleration for a fairly long time, it ends up spinning rapidly and rotates through many revolutions. In more technical terms, if the wheel’s angular acceleration α is large for a long period of time t then the final angular velocity ω and angle of rotation θ are large.
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- 10.5: Angular Momentum and Its Conservation
- Angular momentum is completely analogous to linear momentum. It has the same implications in terms of carrying rotation forward, and it is conserved when the net external torque is zero. Angular momentum, like linear momentum, is also a property of the atoms and subatomic particles.
Thumbnail: The torque caused by the normal force – Fg and the weight of the top causes a change in the angular momentum L in the direction of that torque. This causes the top to precess. (CC-BY-SA-2.5; Xavier Snelgrove ) .