# 8.01x - Lect 19 - Rotating Objects, Moment of Inertia, Rotational KE, Neutron Stars

TLDRThe transcript discusses the principles of rotational motion, including angular velocity, tangential acceleration, and the calculation of kinetic energy in rotating objects. It introduces the concept of moment of inertia and its significance in determining the rotational kinetic energy stored in objects like flywheels. The potential applications of storing and utilizing this energy, particularly in the context of vehicle braking systems and energy management, are explored. The lecture also delves into the rotational kinetic energy of celestial bodies like the sun and the earth, and the Crab Pulsar, highlighting the vast amounts of energy these celestial objects contain and radiate.

###### Takeaways

- π The relationship between linear and angular motion can be understood through equations where x is replaced with ΞΈ, v with Ο, and a with Ξ±.
- π The kinetic energy of a rotating object is given by the formula: 0.5 * I * Ο^2, where I is the moment of inertia and Ο is the angular velocity.
- π« The moment of inertia (I) depends on the mass distribution of the rotating object and can be calculated for various shapes using integral calculus or lookup tables.
- π The parallel axis theorem states that the moment of inertia about an axis parallel to another axis through the center of mass is the moment of inertia about the original axis plus the mass of the object times the square of the distance between the axes.
- π§ The perpendicular axis theorem applies to thin objects and states that the moment of inertia about an axis perpendicular to the object is equal to the sum of the moments of inertia about two mutually perpendicular axes within the object's plane.
- π Flywheels store rotational kinetic energy that can be converted into other forms of energy, such as electricity, and are used in applications like regenerative braking in vehicles.
- π The Crab Pulsar, a neutron star, has a significantly higher rotational kinetic energy compared to the Sun, despite having a smaller moment of inertia due to its compact size.
- π‘ The rotational kinetic energy of celestial bodies like the Earth and the Sun is immense, but not sufficient to be the primary source of their emitted energy.
- π Astrophysical objects like the Crab Pulsar convert their rotational kinetic energy into electromagnetic radiation, such as X-rays and gamma rays.
- π The concept of using the Earth's rotational energy to power global consumption is theoretically possible but practically far-fetched and would have significant consequences.
- π Large-scale flywheels, like those at MIT's Magnet Lab, demonstrate the potential for storing and releasing large amounts of energy for scientific and technological purposes.

###### Q & A

### What is the relationship between tangential acceleration and angular acceleration in circular motion?

-Tangential acceleration is the rate of change of velocity along the circumference of the circle, given by omega dot times R, where omega dot is the rate of change of angular velocity and R is the radius. Angular acceleration, denoted by alpha, is the rate of change of angular velocity, measured in radians per second squared.

### How can equations from linear motion be applied to circular motion?

-Equations from linear motion can be adapted for circular motion by replacing linear variables with their rotational equivalents: replace x with theta (angle), v with omega (angular velocity), and a (linear acceleration) with alpha (angular acceleration). This allows for the application of familiar equations like the one for uniform acceleration in a circular context.

### What is the formula for the kinetic energy of rotation of a disk?

-The kinetic energy of rotation of a disk is given by the formula K = (1/2) * I * omega^2, where I is the moment of inertia of the disk and omega is the angular velocity.

### How is the moment of inertia of a rotating disk calculated?

-The moment of inertia (I) of a rotating disk is calculated by summing the product of each mass element (m_i) and the square of its distance from the axis of rotation (r_i^2). For a solid disk rotating about an axis through its center and perpendicular to the disk, the moment of inertia is (1/2) * m * R^2, where m is the mass and R is the radius of the disk.

### What are the parallel axis and perpendicular axis theorems in the context of moment of inertia?

-The parallel axis theorem states that the moment of inertia about any axis parallel to the axis through the center of mass is equal to the moment of inertia about the central axis plus the mass of the object times the square of the distance (d^2) between the axes. The perpendicular axis theorem applies to thin objects and states that the moment of inertia for rotation about an axis perpendicular to a thin plate is equal to the sum of the moments of inertia about the two principal axes within the plate.

### What is the concept of a flywheel and how is it used to store energy?

-A flywheel is a rotating mechanical device that stores energy in the form of rotational kinetic energy. It can be used to smooth out power delivery in engines, store energy for later use, or in the case of the discussed scenario, convert gravitational potential energy into rotational kinetic energy to be used later for propulsion or other energy needs.

### How does the Department of Energy view the potential of flywheels for energy storage?

-The Department of Energy is taking the concept of flywheels for energy storage seriously. They are considering the installation of flywheels in vehicles to capture and store energy that would otherwise be lost as heat during braking, with the aim of reusing this energy for propulsion or other purposes, potentially improving energy efficiency and reducing costs.

### What is the moment of inertia of the Crab Pulsar and how does it compare to that of the Sun?

-The moment of inertia of the Crab Pulsar is significantly smaller than that of the Sun due to its much smaller radius, despite having a slightly greater mass. The moment of inertia is directly proportional to the square of the radius, so the compact nature of the pulsar results in a much lower value compared to the Sun, even though the Sun has a larger radius and a moment of inertia calculated using the two-fifths mR squared approximation.

### How does the Crab Pulsar generate energy?

-The Crab Pulsar generates energy through the loss of its rotational kinetic energy. As it slows down, it radiates energy in the form of X-rays and gamma rays. The power output of the pulsar, approximately six times ten to the 31st watts, matches the loss of rotational kinetic energy, confirming that the energy it emits comes from its rotation.

### What is the significance of the Crab Pulsar's blinking observation?

-The blinking observation of the Crab Pulsar is significant because it provides a clear indication of the pulsar's rotation frequency. By using a stroboscopic exposure that matches this frequency, astronomers can distinguish the pulsar from other stars that do not blink at the same rate, allowing for the identification and study of the pulsar's properties and behavior.

### How does the Chandra X-ray Observatory contribute to the study of the Crab Pulsar?

-The Chandra X-ray Observatory has provided high-resolution X-ray images of the Crab Pulsar and the surrounding Crab Nebula. These images have revealed the pulsar's emission of X-rays and the presence of a jet, confirming that the energy observed is derived from the pulsar's rotational kinetic energy. The observatory's data has significantly advanced our understanding of the Crab Pulsar and its energetic processes.

###### Outlines

##### π Introduction to Circular Motion and Acceleration

This paragraph introduces the concept of circular motion, where an object with velocity v moves around with angular velocity omega. It explains how the angle theta changes over time and how velocity and acceleration in circular motion differ from linear motion. The paragraph introduces the concept of tangential acceleration and angular acceleration (alpha), and how they relate to linear motion equations when converted using variables theta, omega, and alpha. It also poses a question about the kinetic energy in a rotating disk and sets up the problem for further discussion.

##### π« Kinetic Energy and Moment of Inertia

The paragraph delves into the calculation of kinetic energy in a rotating disk, contrasting it with the linear motion kinetic energy formula. It introduces the concept of moment of inertia, which is crucial for calculating rotational kinetic energy. The moment of inertia is defined as the sum of the product of each mass element and the square of its distance from the axis of rotation. The paragraph also discusses how the moment of inertia depends on the object's shape and the axis of rotation, providing examples for a solid disk, sphere, and rod. It mentions the parallel and perpendicular axis theorems, which help in calculating moments of inertia for different scenarios.

##### π Energy Conservation in Vehicles

This section discusses the potential for energy conservation in vehicles, particularly in the context of rotational kinetic energy. It uses the example of a car descending a mountain road, where the kinetic energy generated is usually lost as heat through braking. The paragraph proposes the idea of using a flywheel to convert gravitational potential energy into rotational kinetic energy, which can then be used later, potentially reducing fuel consumption. It provides a hypothetical calculation for a flywheel's rotational speed and frequency based on the car's mass and the height difference, and suggests that this technology could be economically viable and environmentally friendly.

##### π Practical Applications and Theoretical Considerations

The paragraph explores the practical applications of flywheels in energy storage and their potential use in urban driving conditions. It mentions the interest of the Department of Energy in this technology and provides an example of a flywheel energy management power plant. The paragraph also discusses the use of flywheels in MIT's Magnet Lab to generate strong magnetic fields, highlighting the conversion between mechanical and magnetic energy. The speaker shares a personal anecdote about his childhood thoughts on energy conservation and introduces the concept of rotational kinetic energy in celestial bodies like the sun and the earth.

##### π Rotational Kinetic Energy in the Cosmos

This section focuses on the immense rotational kinetic energy stored in celestial bodies, such as the sun and the earth, and the theoretical possibility of harnessing this energy. It presents calculations for the rotational kinetic energy of the sun and the earth and contrasts these with the world's energy consumption. The paragraph then introduces the Crab Pulsar, a neutron star with a significantly higher rotational kinetic energy output compared to the sun, which it radiates as x-rays and gamma rays. The speaker explains that the pulsar's energy output is directly linked to its loss of rotational kinetic energy.

##### π Observational Evidence and Future Speculation

The paragraph discusses the observational evidence of the Crab Pulsar's energy output, including the use of stroboscopic photography to identify the pulsar and the recent images captured by the Chandra X-ray Observatory. It describes the pulsar's jet and the conversion of rotational kinetic energy into x-ray emissions. The speaker also mentions the role of MIT and Cambridge in operating the Chandra Science Center and the significant contributions of MIT scientists to the project. The paragraph concludes with a teaser for the next lecture.

###### Mindmap

###### Keywords

##### π‘Angular Velocity

##### π‘Centripetal Acceleration

##### π‘Moment of Inertia

##### π‘Kinetic Energy of Rotation

##### π‘Flywheel

##### π‘Parallel Axis Theorem

##### π‘Perpendicular Axis Theorem

##### π‘Crab Pulsar

##### π‘Astrophysical Applications

##### π‘Energy Conversion

###### Highlights

The concept of rotational motion is introduced, comparing it to linear motion and discussing the transfer of equations between the two domains.

The relationship between tangential acceleration, angular acceleration, and centripetal acceleration is explained.

The method of calculating kinetic energy for a rotating object, specifically a disk, is detailed, introducing the concept of moment of inertia.

The moment of inertia is defined and its importance in calculating the kinetic energy of rotation is emphasized.

The parallel axis theorem is introduced, providing a method to calculate the moment of inertia for different axes of rotation.

The perpendicular axis theorem is explained, applicable for calculating the moment of inertia for thin objects like plates.

The application of flywheels in storing and utilizing rotational kinetic energy is discussed, with examples from real-world scenarios.

An example of a car utilizing a flywheel to convert gravitational potential energy into rotational kinetic energy is provided.

The potential of using the earth's rotational kinetic energy for global energy consumption is humorously proposed and dismissed as impractical.

The Crab Pulsar is introduced as a celestial object that derives its radiated energy from rotational kinetic energy.

The Crab Pulsar's rotational kinetic energy is calculated and compared to the sun's, showing a significant difference.

The Chandra X-ray Observatory's role in capturing detailed images of the Crab Nebula and the pulsar is highlighted.

MIT's involvement in the Chandra X-ray Observatory and its contributions to the field of astrophysics are mentioned.

The concept of converting linear motion to rotational kinetic energy and vice versa is demonstrated using a toy car with a flywheel.

The potential energy lost during braking is highlighted as a significant source of wasted energy that could theoretically be stored in flywheels.

The practicality and economic feasibility of using flywheels in vehicles are discussed, with the Department of Energy showing interest in such technology.

###### Transcripts

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