# Describing the motion of a linked body?

by greswd
 P: 147 A general example of a linked body is an object that is made up of rigid bodies joined together by pivots, such as hinges and ball-socket joints. If the pivots are frictionless, and I apply a force to one rigid body, how can I go about describing the subsequent angular acceleration of the individual rigid bodies, and also the entire linked body about its barycenter?
 Mentor P: 15,670 I would use the Lagrangian approach. Even for something as simple as a double pendulum it winds up being easier.
 Sci Advisor P: 2,448 Which, with exception of a few simple cases, will usually produce something that can only be solved numerically. So if you hope for a closed-form expression, forget about it. But if you need it for a simulation, do what DaleSpam suggested. Write down the Lagrangian with an undetermined Lagrange Multiplier for every joint or other constraint you have. That will give you a system of equations for each [itex]\ddot{q_i}[/tex] to be solved for each time step, and then you can use Verlet or Runge-Kutta methods to integrate these.
P: 147

## Describing the motion of a linked body?

 Quote by K^2 Which, with exception of a few simple cases, will usually produce something that can only be solved numerically. So if you hope for a closed-form expression, forget about it. But if you need it for a simulation, do what DaleSpam suggested. Write down the Lagrangian with an undetermined Lagrange Multiplier for every joint or other constraint you have. That will give you a system of equations for each [itex]\ddot{q_i}[/tex] to be solved for each time step, and then you can use Verlet or Runge-Kutta methods to integrate these.
So it's possible to get a numerical solution to any degree of accuracy? I've always wondered how mathematicians definitively prove that the system is inherently chaotic and that no closed form solutions exist.

But even if a pattern exists it's difficult to find because the subsequent motion is so highly dependent on initial conditions.
 P: 147 what about a very simple compound body? Just one mass, floating in deep space, with a rod pivoted to it.
P: 2,448
 Quote by greswd So it's possible to get a numerical solution to any degree of accuracy?
Not in general. There are going to be special cases where you can, but in general, these things tend towards chaos.

If you have just two masses connected by a single joint, you can use conservation of momentum and angular momentum to greatly reduce degrees of freedom, giving you a closed form solution. Everything past that would require some approximations, I believe.
P: 147
 Quote by K^2 Not in general. There are going to be special cases where you can, but in general, these things tend towards chaos. If you have just two masses connected by a single joint, you can use conservation of momentum and angular momentum to greatly reduce degrees of freedom, giving you a closed form solution. Everything past that would require some approximations, I believe.
Thanks. So in certain cases things are non-chaotic, but it's generally difficult to prove whether system is definitely chaotic and/or lacks a closed form solution.
 Mentor P: 15,670 If you are interested in chaos, I would start with a double pendulum. It is relatively easy to solve, and it is well studied. Here is a good start: http://en.wikipedia.org/wiki/Double_pendulum
 P: 147 Hmmm, I am interested in finding out how mathematicians prove that a system is chaotic and that no symbolic solutions exist.
 P: 147 For instance, the three body problem of Sun-Moon-Earth might have a definite closed form solution that we just cannot find.
Mentor
P: 15,670
 Quote by greswd Hmmm, I am interested in finding out how mathematicians prove that a system is chaotic and that no symbolic solutions exist.
These are two somewhat different properties. A system can be non-chaotic and still not have a symbolic solution.

However, I don't think that "no symbolic solutions" is something that is actually mathematically proven. It is simply that we don't know any such solutions. It also depends critically on what functions are admissible in your set of symbolic colutions.
P: 147
 Quote by DaleSpam A system can be non-chaotic and still not have a symbolic solution.
What kind of system is described as such?

 Quote by DaleSpam However, I don't think that "no symbolic solutions" is something that is actually mathematically proven. It is simply that we don't know any such solutions. It also depends critically on what functions are admissible in your set of symbolic colutions.
Interesting, it sort of opens the possibility that someday someone might solve the Sun-Earth-Moon problem.

Like the CMI problems.

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