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CH 10: Rotation

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Rotational Kinetic Energy Kinetic energy describes the amount of energy an object has when it is moving. We have discussed translational motion, but we must also consider rotational motion. Let us look at the kinetic energy of a system of particles that is rotating about an arbitrary point. We begin by examining the kinetic energy of one particle of that system. K i – Kinetic energy of i th particle v i – Tangential velocity of i th particle It is more convenient to look at the angular velocity for a system of rotating objects, especially if we assume they all have the same angular velocity. If we look at the entire system, we must sum the kinetic energies for each particle. Where K R – Rotational kinetic energy [J] I – Rotational Inertia [kg m 2 ] We must consider the distribution of mass as opposed to just the mass.

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Rotational Inertia (Moment of Inertia) The rotational inertia is the inertia of a rotating object. This determines how hard it is to change the motion of a rotating object. The mass of the object coupled with the distance from the point of rotation defines the rotational inertia. The more mass and the greater the distance from the point of rotation, the harder it is to change the rotation of the object. Discrete particles Continuous object I – Rotational Inertia r – the distance from the axis of rotation

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Two cylinders of the same size and mass roll down an incline. Cylinder A has most of its weight concentrated at the rim, while cylinder B has most of its weight concentrated at the center. Which reaches the bottom of the incline first? 1. A 2. B 3. Both reach the bottom at the same time.

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A solid disk and a ring roll down an incline. The ring is slower than the disk if 1. m ring = m disk, where m is the inertial mass. 2. r ring = r disk, where r is the radius. 3. m ring = m disk and r ring = r disk. 4. The ring is always slower regardless of the relative values of m and r.

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Example: Determine the rotational inertia of a cylinder about its central axis. R M L x y z Total volume of cylinder Total mass of cylinder Rotational Inertia of a solid cylinder rotating about its longitudinal axis.

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Parallel-Axis Theorem The rotational inertial of different objects through an axis of symmetry was shown for several objects. If the rotation axis is shifted away from an axis of symmetry the calculation becomes more difficult. A simple method, called the parallel axis theorem, was devised for situations where the rotation axis was shifted some distance from the symmetry axis. The symmetry axis is any axis that passes through the center of mass. I – rotational inertia I CM – Rotational inertia for a rotation axis that passes through the center of mass M – Total mass of the object D – Distance the axis has been shifted by The new rotation axis must be parallel to the symmetry axis that is being used to define I CM.

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