Coupling only to the center of mass degrees of freedom?

In summary, the conversation discusses the search for an interaction that only affects the center of mass degrees of freedom in a composite system. The possibility of using electromagnetic fields or other methods is considered, but it is ultimately suggested that homogeneous gravitational fields may be the best option. The possibility of using weak and strong interactions is also ruled out, and the Mössbauer effect and light scattering are mentioned as potential options.
  • #1
JK423
Gold Member
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I am interested in the following scenario.

Suppose that you have a composite system, e.g. ion, atom or whatever you want. This system will have internal degrees of freedom (d.o.f.), i.e. relative positions and momenta, and also center of mass d.o.f..What i am looking for is to find an interaction (using e.g. E/M fields or something else(?)) that couples only to the center of mass d.o.f. and not to the internal ones. I'm not sure if that's even possible.
So basically i want to push the system around in space without giving energy to each constituents.

Do you know any way of doing that?

Thanks
 
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  • #2
Homogeneous gravitational fields. This is the best, and the only general method.
 
  • #3
Thanks mfb!

I'm interested in non-zero proper acceleration, and for a free falling particle that would be zero. So i don't think that gravitation is suitable.

You think it's not possible without gravity?
 
  • #4
You need some force which is proportional to mass, for every particle in the setup.

The weak and strong interaction do not have a sufficient range to provide such an interaction as soon as your system is larger than a nucleus. The classic electromagnetic interaction cannot do that either, as electrons and protons (or nuclei) have opposite charges. The Mössbauer effect provides momentum to the whole crystal without exciting any internal degrees of freedom, and scattering of light is similar, but I don't know if that counts.
 

1. What does "coupling only to the center of mass degrees of freedom" mean?

"Coupling only to the center of mass degrees of freedom" refers to a scientific principle in which a system or object is affected only by external forces acting on its center of mass. This means that the overall motion of the system is determined by the forces acting on its center of mass, rather than individual components within the system.

2. Why is it important to consider the center of mass when studying systems?

The center of mass is a crucial concept in physics as it allows us to simplify the analysis of complex systems by treating them as a single point. This simplification allows for easier calculations and predictions of the overall motion of the system.

3. How is the center of mass determined in a system?

The center of mass is determined by finding the average location of all the mass in a system. For a system with discrete particles, it can be calculated by taking the sum of the products of each particle's mass and its distance from a chosen reference point, divided by the total mass of the system. For continuous objects, it can be found using integrals.

4. What are some examples of systems that primarily couple to the center of mass degrees of freedom?

Systems that primarily couple to the center of mass degrees of freedom include a pendulum, a seesaw, and a rolling ball. In these systems, the overall motion is determined by the forces acting on their center of mass, rather than the individual components within them.

5. What are the advantages of using the center of mass concept in physics?

The center of mass concept allows for a simpler analysis of complex systems and can greatly simplify calculations and predictions of motion. It also provides a more intuitive understanding of how external forces affect a system as a whole, rather than individual components within the system.

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