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Introduction to Symmetry Analysis. Chapter 4 -Classical Dynamics. Brian Cantwell Department of Aeronautics and Astronautics Stanford University. Consider a spring-mass system. Equation of motion. Energy is conserved. The sum of kinetic energy,. is called the Hamiltonian. - PowerPoint PPT Presentation
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Stanford University Department of Aeronautics and Astronautics
Introduction to Symmetry Analysis
Brian CantwellDepartment of Aeronautics and Astronautics
Stanford University
Chapter 4 -Classical Dynamics
Stanford University Department of Aeronautics and Astronautics
Consider a spring-mass system
H =1
2mdxdt
⎛⎝⎜
⎞⎠⎟
2
+12kx2 =T +V
Equation of motion
Energy is conserved. The sum of kinetic energy,
is called the Hamiltonian.
Stanford University Department of Aeronautics and Astronautics
Dynamical systems that conserve energy follow a path in phase space that corresponds to an extremum in a certain integral of the coordinates and velocities called the action integral.
There is a very general approach to problems of this type called Lagrangian dynamics.
Usually the extremum is a minimum and this theory is often called the principle of least action.
The kernel of the integral is called the Lagrangian. Typically,
L =T −V
Stanford University Department of Aeronautics and Astronautics
Apply a small variation in the coodinates and velocities.
Consider
Stanford University Department of Aeronautics and Astronautics
At an extremum in S the first variation vanishes.
Using
Integrate by parts.
At the end points the variation is zero.
Stanford University Department of Aeronautics and Astronautics
The Lagrangian satisfies the Euler-Lagrange equations.
Spring mass system
The Euler-Lagrange equations generate
Stanford University Department of Aeronautics and Astronautics
The Two-Body Problem
The Lagrangian of the two-body system is
Stanford University Department of Aeronautics and Astronautics
Set the origin of coordinates at the center-of-mass of the two points
Insert (4.80) into (4.79).
where r = r1 - r2.
Stanford University Department of Aeronautics and Astronautics
In terms of the center-of-mass coordinates
where the reduced mass is
Stanford University Department of Aeronautics and Astronautics
Equations of motion generated by the Euler-Lagrange equations
The Hamiltonian is
Stanford University Department of Aeronautics and Astronautics
The motion of the particle takes place in a plane and so it is convenient to express the position of the particle in terms of cylindrical coordinates.
The Hamiltonian is the total energy which is conserved
The equations of motion in cylindrical coordinates simplify to
Angular momentum is conserved (Kepler’s Second Law)
Stanford University Department of Aeronautics and Astronautics
Use the Hamiltonian to determine the radius
Integrate
Determine the angle from conservation of angular momentum
Stanford University Department of Aeronautics and Astronautics
The particle moving under the influence of the central field is constrained to move in an annular disk between two radii.
Stanford University Department of Aeronautics and Astronautics
Kepler’s Two-Body ProblemLet
Lagrangian
Generalized momenta
Stanford University Department of Aeronautics and Astronautics
Gravitational constant
Equations of motion
In cylindrical coordinates
Stanford University Department of Aeronautics and Astronautics
The two-body Kepler solution
Relationship between the angle and radius
or
Stanford University Department of Aeronautics and Astronautics
Trajectory in Cartesian coordinates
Stanford University Department of Aeronautics and Astronautics
Semi-major and semi-minor axes
Apogee and perigee
Stanford University Department of Aeronautics and Astronautics
Orbital period
Area of the orbit
Equate (4.108) and (4.109)
Stanford University Department of Aeronautics and Astronautics
Invariant group of the governing equations