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**Chapter 11 Angular Momentum; General Rotation**

10-9 Rotational Plus Translational Motion; Rolling 11-1 Angular Momentum—Objects Rotating About a Fixed Axis 11-2 Vector Cross Product; Torque as a Vector 11-3Angular Momentum of a Particle 11-4 Angular Momentum and Torque for a System of Particles; General Motion 11-5 Angular Momentum and Torque for a Rigid Object 11-6 Conservation of Angular Momentum

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**10-9 Rotational Plus Translational Motion; Rolling**

In (a), a wheel is rolling without slipping. The point P, touching the ground, is instantaneously at rest, and the center moves with velocity (rotation and translation) In (b) the same wheel is seen from a reference frame where C is at rest. Now point P is moving with velocity– .(Pure rotation). The linear speed of the wheel is related to its angular speed: (a) Figure Caption: (a) A wheel rolling to the right. Its center C moves with velocity v. Point P is at rest at this instant. (b) The same wheel as seen from a reference frame in which the axle of the wheel C is at rest—that is, we are moving to the right with velocity v. relative to the ground. Point P, which was at rest in (a), here in (b) is moving to the left with velocity –v as shown. (See also Section 3-9 on relative velocity.) (b)

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**11-1 Angular Momentum—Objects Rotating About a Fixed Axis**

The rotational analog of linear momentum is angular momentum, L: Then the rotational analog of Newton’s second law is: This form of Newton’s second law is valid even if I is not constant.

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**11-1 Angular Momentum—Objects Rotating About a Fixed Axis**

In the absence of an external torque, angular momentum is conserved: More formally, The total angular momentum of a system remains constant if the net external torque acting on the system is zero.

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Ice Skater Movie

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**11-1 Angular Momentum—Objects Rotating About a Fixed Axis**

This means: Therefore, if an object’s moment of inertia changes, its angular speed changes as well. Figure 11-1: A skater doing a spin on ice, illustrating conservation of angular momentum: (a) I is large and ω is small; (b) I is smaller so ω is larger. Figure 11-2: A diver rotates faster when arms and legs are tucked in than when they are outstretched. Angular momentum is conserved.

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**11-1 Angular Momentum—Objects Rotating About a Fixed Axis**

Example 11-1: Object rotating on a string of changing length. A small mass m attached to the end of a string revolves in a circle on a frictionless tabletop. The other end of the string passes through a hole in the table. Initially, the mass revolves with a speed v1 = 2.4 m/s in a circle of radius R1 = 0.80 m. The string is then pulled slowly through the hole so that the radius is reduced to R2 = 0.48 m. What is the speed, v2, of the mass now? Solution: Conservation of angular momentum gives the speed as 4.0 m/s.

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**11-1 Angular Momentum—Objects Rotating About a Fixed Axis**

Example 11-2: Clutch. A simple clutch consists of two cylindrical plates that can be pressed together to connect two sections of an axle, as needed, in a piece of machinery. The two plates have masses MA = 6.0 kg and MB = 9.0 kg, with equal radii R0 = 0.60 m. They are initially separated. Plate MA is accelerated from rest to an angular velocity ω1 = 7.2 rad/s in time Δt = 2.0 s. Calculate (a) the angular momentum of MA, and (b) the torque required to have accelerated MA from rest to ω1. (c) Next, plate MB, initially at rest but free to rotate without friction, is placed in firm contact with freely rotating plate MA, and the two plates both rotate at a constant angular velocity ω2, which is considerably less than ω1. Why does this happen, and what is ω2? Solution: a. The angular momentum is 7.8 kg·m2/s. b. The torque is the change in angular momentum divided by the time, 3.9 m·N. c. Angular momentum is conserved (this is a rotational collision), so the new angular speed is 2.9 rad/s.

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**11-1 Angular Momentum—Objects Rotating About a Fixed Axis**

Angular momentum is a vector; for a symmetrical object rotating about a symmetry axis it is in the same direction as the angular velocity vector. Figure (a) A person on a circular platform, both initially at rest, begins walking along the edge at speed v. The platform, assumed to be mounted on friction-free bearings, begins rotating in the opposite direction, so that the total angular momentum remains zero, as shown in (b).

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**Direction of Angular Momentum**

A right hand rule: Using your right hand, 1) curl your fingers in the direction of the rotation 2) Your thumb points in the direction of the angular momentum.

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**11-2 Vector Cross Product; Torque as a Vector**

The vector cross product is defined as: The direction of the cross product is defined by a right-hand rule: Figure The vector C = A x B is perpendicular to the plane containing A and B; its direction is given by the right-hand rule.

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**11-2 Vector Cross Product; Torque as a Vector**

The cross product can also be written in determinant form:

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