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  5. Precession of a gyroscope

11.4 Precession of a gyroscope

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By the end of this section, you will be able to:
  • Describe the physical processes underlying the phenomenon of precession
  • Calculate the precessional angular velocity of a gyroscope

[link] shows a gyroscope , defined as a spinning disk in which the axis of rotation is free to assume any orientation. When spinning, the orientation of the spin axis is unaffected by the orientation of the body that encloses it. The body or vehicle enclosing the gyroscope can be moved from place to place and the orientation of the spin axis will remain the same. This makes gyroscopes very useful in navigation, especially where magnetic compasses can’t be used, such as in manned and unmanned spacecraft, intercontinental ballistic missiles, unmanned aerial vehicles, and satellites like the Hubble Space Telescope.

A drawing of a gyroscope, consisting of a disk that can spin on an shaft, perpendicular to the plane of the disk and through its center. Two rings surround the gyroscope. One is attached to the shaft above and below the disk, and the other is attached to the first ring and is in the plane of the disk so that this second ring is concentric with the disk.
A gyroscope consists of a spinning disk about an axis that is free to assume any orientation.

We illustrate the precession    of a gyroscope with an example of a top in the next two figures. If the top is placed on a flat surface near the surface of Earth at an angle to the vertical and is not spinning, it will fall over, due to the force of gravity producing a torque acting on its center of mass. This is shown in [link] (a). However, if the top is spinning on its axis, rather than topple over due to this torque, it precesses about the vertical, shown in part (b) of the figure. This is due to the torque on the center of mass, which provides the change in angular momentum.

Figure a: An x y z coordinate system is show, with x out of the page, y to the right, and z up. The origin is point O. A top is shown with its point at the origin and its axis tilted away from the vertical z axis. The axis of the top is the line O O prime. The vector r extends from the origin to the center of the mass, labeled as C M, of the top. The force M g acts downward at the center of mass. The torque about the origin is equal to vector r crossed with M vector g. This torque is a vector in the x y plane, perpendicular to the r vector. Figure b: The x y z coordinate and the top are shown. The top is again tilted away from the z axis and is spinning rapidly counterclockwise about the O O prime axis as viewed from above. The precession of the top traces a counterclockwise circle as viewed from above, centered on the z axis. The cone swept by the precession of the top is indicated using dashed lines.
(a) If the top is not spinning, there is a torque r → × M g → about the origin, and the top falls over. (b) If the top is spinning about its axis O O ′ , it doesn’t fall over but precesses about the z- axis.

[link] shows the forces acting on a spinning top. The torque produced is perpendicular to the angular momentum vector. This changes the direction of the angular momentum vector L → according to d L → = τ → d t , but not its magnitude. The top precesses around a vertical axis, since the torque is always horizontal and perpendicular to L → . If the top is not spinning, it acquires angular momentum in the direction of the torque, and it rotates around a horizontal axis, falling over just as we would expect.

An x y z coordinate system is show, with x out of the page, y to the right, and z up. The origin is point O. A top is shown with its point at the origin and its axis tilted by an angle theta away from the vertical z axis, clockwise as we view it. The vector r extends from the origin to the center of the mass, labeled as C M, of the top. The force M g acts downward at the center of mass. The torque, tau, about the origin is equal to vector r crossed with M vector g. This torque is a vector in the x y plane, perpendicular to the r vector, into the page. The angular velocity, omega, of the top is counterclockwise as viewed from above. The angular momentum, L, is in the same direction as the r vector, tilted up along the axis of the top. The circle traced by the precession of the top is shown as a horizontal circle above the top. The precession angular velocity omega sub p is counterclockwise as viewed from above. The radius of the precession circle is L sine theta. The vector d L is tangent to the circle, pointing into the page, and is equal to vector tau d t. The triangle formed L sine theta and d L is shown, and the angle across from d L is labeled as d phi.
The force of gravity acting on the center of mass produces a torque τ → in the direction perpendicular to L → . The magnitude of L → doesn’t change but its direction does, and the top precesses about the z -axis.

We can experience this phenomenon first hand by holding a spinning bicycle wheel and trying to rotate it about an axis perpendicular to the spin axis. As shown in [link] , the person applies forces perpendicular to the spin axis in an attempt to rotate the wheel, but instead, the wheel axis starts to change direction to her left due to the applied torque.

In figure a, a woman, facing the viewer, is holding a spinning bike wheel of radius r by the axle. The wheel is so that the angular velocity omega and angular momentum L are along the axis of rotation of the wheel, to her left (the viewer’s right.) That is, the motion of the wheel is such that the bottom of the wheel is moving toward her (into the page.) The direction of the force F applied by her left hand is shown downward and that by her right hand in upward direction. The torque tau is toward her (into the page.) In figure b, addition of two vectors L and delta-L, which is parallel to torque tau, is shown. The resultant of the two vectors is labeled as L plus delta L. The direction of rotation, omega sub p, is counterclockwise as viewed from above.
(a) A person holding the spinning bike wheel lifts it with her right hand and pushes down with her left hand in an attempt to rotate the wheel. This action creates a torque directly toward her. This torque causes a change in angular momentum Δ L → in exactly the same direction. (b) A vector diagram depicting how Δ L → and L → add, producing a new angular momentum pointing more toward the person. The wheel moves toward the person, perpendicular to the forces she exerts on it.
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Source:  OpenStax, University physics volume 1. OpenStax CNX. Sep 19, 2016 Download for free at http://cnx.org/content/col12031/1.5
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