# Lecture 3 | Modern Physics: Classical Mechanics (Stanford) | Summary and Q&A

## Summary

In this video, the speaker discusses the structure of the laws of physics and introduces the principle of least action. They explain that the laws of physics have a common form, which is the principle of least action. They also introduce the concept of Lagrangian, which is the difference between kinetic and potential energy, and show how it can be used to derive the equations of motion for a system.

## Questions & Answers

### Q: What is the principle of least action?

The principle of least action is a fundamental concept in physics that states that the true trajectory of a physical system is the one that minimizes the action, where action is defined as the integral of the Lagrangian along the trajectory.

### Q: What is the Lagrangian?

The Lagrangian is a function that describes the difference between the kinetic and potential energy of a system. It is defined as L = T - U, where T is the kinetic energy and U is the potential energy. The Lagrangian is used to derive the equations of motion for a system.

### Q: How does the principle of least action relate to the laws of physics?

The laws of physics can be derived from the principle of least action. The principle states that the true trajectory of a physical system is the one that minimizes the action. By varying the action with respect to the trajectory, we can derive the equations of motion for the system, which describe how the system evolves over time.

### Q: How do the Lagrangian and the principle of least action relate to each other?

The Lagrangian is used to define the action of a system. The action is the integral of the Lagrangian along the trajectory of the system. The principle of least action states that the true trajectory of a system is the one that minimizes the action. In other words, the true trajectory is the one that makes the action as small as possible.

### Q: Can you give an example of how the principle of least action is used to derive the equations of motion?

Let's consider a simple example of a particle moving in one dimension. The Lagrangian for this system is L = (1/2)m(x_dot)^2 - U(x), where m is the mass of the particle, x_dot is the velocity of the particle, and U(x) is the potential energy. By varying the action with respect to the position of the particle, we can derive the equation of motion as m(x_double_dot) = -dU/dx, which is Newton's second law.

### Q: How does the principle of least action apply to systems with multiple particles?

The principle of least action can be applied to systems with multiple particles by considering the Lagrangian that describes the entire system. The Lagrangian is the sum of the Lagrangians for each individual particle. By varying the action with respect to the coordinates of each particle, we can derive the equations of motion for the system as the time derivative of the momentum of each particle is equal to the force acting on it.

### Q: What are the Euler-Lagrange equations?

The Euler-Lagrange equations are a set of differential equations that are derived from the principle of least action. They describe the equations of motion for a system. In general, the Euler-Lagrange equations state that the time derivative of the derivative of the Lagrangian with respect to the velocity of a coordinate is equal to the derivative of the Lagrangian with respect to the coordinate.

### Q: Why is the Lagrangian subtracted from the potential energy?

The Lagrangian is subtracted from the potential energy to ensure that the principle of least action is satisfied. By subtracting the Lagrangian, which represents the kinetic energy, from the potential energy, we ensure that the true trajectory of a system is the one that minimizes the action.

### Q: Can the principle of least action be applied to other areas of physics besides mechanics?

Yes, the principle of least action can be applied to other areas of physics besides mechanics. It is a fundamental concept that can be used to describe the behavior of physical systems in various fields, such as electromagnetism, quantum mechanics, and even general relativity. In each case, the Lagrangian is defined differently based on the specific system, but the principle of least action still holds.

### Q: Does the Lagrangian depend on the choice of coordinates or variables used to describe the system?

Yes, the Lagrangian can depend on the choice of coordinates or variables used to describe the system. The Lagrangian is a function of the coordinates and their derivatives, and the choice of coordinates can affect the form of the Lagrangian. However, the equations of motion derived from the Lagrangian are invariant under coordinate transformations, meaning that they hold true regardless of the choice of coordinates.

## Takeaways

The principle of least action is a fundamental concept in physics that states that the true trajectory of a physical system is the one that minimizes the action, which is defined as the integral of the Lagrangian along the trajectory. The Lagrangian is a function that describes the difference between the kinetic and potential energy of a system and is used to derive the equations of motion for the system. The principle of least action can be applied to various areas of physics and is a powerful tool for understanding the behavior of physical systems. The Lagrangian and the principle of least action provide a unified framework for describing the laws of physics and can be used to derive the equations of motion for a wide range of systems.