Types of Precession: Gyroscopic, Astronomical & Relativistic
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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FAQs on What Is Precession in Physics?
1. What is precession in physics?
In physics, precession is the change in the orientation of the rotational axis of a spinning object. Instead of falling over when subjected to an external force or torque, a spinning body's axis will sweep out a cone shape. This 'wobbling' motion of the axis itself is what we call precession. It occurs because the applied torque changes the direction of the object's angular momentum vector.
2. What are some common examples of precession?
Precession can be observed in various everyday and astronomical scenarios. The most common examples include:
- A Spinning Top: A toy top wobbles in a circle as it spins, demonstrating precession due to the torque created by gravity.
- A Gyroscope: When a gyroscope is spinning, its axis resists changes in orientation and will precess in response to an external torque.
- Earth's Axial Precession: The Earth's rotational axis slowly wobbles, completing a full cycle approximately every 26,000 years. This is caused by the gravitational pull of the Sun and Moon.
3. Why does a spinning top precess instead of just falling over?
A spinning top doesn't just fall over because of the interplay between torque and angular momentum. Gravity exerts a torque that tries to tip the top over. However, because the top has significant angular momentum from its spin, this torque doesn't topple it. Instead, it causes the angular momentum vector (which points along the spin axis) to change direction. This continuous change in direction results in the conical 'wobble' of precession, keeping the top upright as long as it spins fast enough.
4. What are the main types of precession?
Precession is primarily categorized into two types based on the presence or absence of external forces:
- Torque-Induced Precession: This is the most common type, occurring when an external torque is applied to a rotating body. The precession of a spinning top due to gravity is a classic example.
- Torque-Free Precession: This occurs in a rotating body that is not perfectly spherical, even without any external torque. It happens because the axis of rotation and the body's axis of symmetry are not aligned.
5. How is precession different from nutation?
Precession and nutation are related but distinct motions. Precession is the slow, large-scale conical wobble of a spinning body's axis. In contrast, nutation is a smaller, faster rocking or nodding motion that is superimposed on the precessional path. If you imagine the axis tracing a large circle due to precession, nutation would be the small, wavy variations along the line of that circle.
6. What is the significance of precession in astronomy?
In astronomy, the precession of the equinoxes (Earth's axial precession) is highly significant. This 26,000-year cycle, caused by the gravitational forces of the Sun and Moon on Earth's equatorial bulge, changes the apparent position of stars over long periods. A major consequence is that our 'North Star' changes over time. Currently, it is Polaris, but in about 12,000 years, it will be Vega. This phenomenon is crucial for long-term astronomical calculations and understanding past celestial observations.
7. How is the principle of precession used in real-world technology like MRI?
The principle of precession is fundamental to Magnetic Resonance Imaging (MRI) technology. In an MRI machine, a powerful magnetic field is used to align the atomic nuclei (specifically, protons) in the body's water molecules. These nuclei naturally precess. Radio waves are then applied at a specific frequency to knock them out of alignment. When the radio waves are turned off, the nuclei 'relax' back to their original state, emitting signals that are detected by the MRI scanner. These signals are used to create detailed images of organs and soft tissues.
