Precession

Precession meaning in physics is the change in orientation of a rotating body with respect to its rotational axis. Precession in a sentence can be defined as the rotational movement in the rotational axis of a rotating object. The phenomena of precession are closely associated with the action of a gyroscope or a spinning top. It refers to the slow rotation of the axis of rotation of a rotating or spinning object about a line intersecting the spinning axis. The movement of astronomical objects on their orbits and the slow and circular movement of a spinning top can be referred to as precessional motion. 

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Types of Precessional Motion

In physics, precession movement is basically of two types: torque-free and torque induced. Let us study in detail both the precessional motions.

Torque free Precession: In precession, torque-free signifies that no external force or torque is applied on the rotating object. In this kind of precession movement, the angular momentum is constant, but the angular velocity changes with time. The change in the body’s angular velocity in torque-free precession is brought about by the moment of inertia. 

The following formula can calculate the torque-free precessional motion of an object spinning about an axis:

ωp = Isωs / Ip cos (α)

Where ωp denotes the precession rate, ωs denotes the spin rate about the axis of symmetry and Is denotes the moment of inertia along the axis. Here, α denotes the angle between the moment of inertia and axis symmetry.

Torque Induced or Gyroscopic Precession: In physics, gyroscopic precession definition is the phenomenon in which the axis of a gyroscope or any other spinning object describes a cone in space when an external force is applied on induced into the object. The phenomena of torque induced precession can be observed primarily on spinning toy tops and gyroscope, and this is why it is named gyroscopic precession.

The torque induced or gyroscopic precession can be explained by understanding the movement of a spinning top, which is discussed in detail below:

In a spinning top, the weight of the top acts downwards from its centre of mass when it is in a rotational motion. The normal force of the ground acting upon the top pushes it when it comes in contact. The weight and the force of the ground act on the spinning top to produce torque and bring about precession. 

Newtonian Precession 

According to classical Newtonian theory, precession can be defined as the change of angular velocity and angular momentum produced by torque. The equation which relates the torque to the change of angular momentum is as follows:

T = dL/dt

Here, L represents the angular momentum, and t represents the torque. 

Relativistic Theories

As a correction to the Newtonian precession theory of gyroscope, the theory of relativity gives three corrections, namely;

• Thomas Precession

• De Sitter Precession

• Lense-Thirring Precession

Let us discuss all three corrections.

Thomas Precession: The Thomas precession is given by Llewellyn Thomas for an object that is being accelerated along a curved path. The Thomas precession can be defined as a motion of the spin axis of an electron caused by the interaction between the electron and the electric field of the nucleus of an atom. 

De Sitter Precession: The De Sitter Precession is a general relativistic correction given for the classic Newtonian precession theory. This correction is given by William de Sitter and is known as the Geodetic effect or Geodetic precession. This theory represents the effect of the curvature of spacetime given by the general relativity theory. The theory explains the precession movement of a spinning body near a large non-rotating mass. 

Lense-Thirring Precession: It is also a general relativistic correction given to the Newtonian precession theory. The correction explains the movement of a spinning object on a curved path with a large rotating mass nearby. This theory is also called the gravitomagnetic frame-digging effect. 

Orbital Precession

In astronomy, precession has a different meaning. In astronomy, precession meaning can be defined as the slow changes that occur in heavenly bodies, and it is also called orbital precession. An example of precession in astronomy can be the steady change in the orientation of the earth’s axis of rotation, also known as the precession of equinoxes.

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