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While studying the concept of center of mass, i read about internal forces as the force applied by particles of a system on each-other but is it necessary for all systems to have internal forces, can there be a system of several particles with no internal force acting ?

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Low energy photons are massless and chargeless, and you can in principle have a system of low energy photons that don't interact via any internal forces. So that's one example.

Shirish Kulhari
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can there be a system of several particles with no internal force acting?

I suppose all particles have some form of interaction (at the very least gravitational), but there are many scenarios where this can be ignored. For example, when treating a system of particles as an ideal gas, we assume the particles are non-interacting (other than the occasional elastic collision, but otherwise we assume no interactions).

s it necessary for all systems to have internal forces?

No, it isn't necessary. The only time you should consider the internal forces of a system is if they are actually present and relevant for the analysis you are performing.

BioPhysicist
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    I don‘t think the example of an ideal gas is correct. Particles do interact by bouncing off from each other. Otherwise you could never reach Maxwell-Boltzmann-distribution when you start with any other initial velocity distribution. – Hartmut Braun Dec 28 '19 at 07:53
  • @HartmutBraun Fixed – BioPhysicist Dec 28 '19 at 12:04
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Is it important for a system of two or multiple particles to have internal force acting on them?

The photon example is a good example of particles existing in a "system" without interacting with each other. But they do not form a bound state, because of this. In addition to have a continuous "system" with photons one needs the photons interacting with matter, as for example in this video here. .

So it depends on your definition of a "system".

For sure particles that have no forces between them cannot form a system of bound particles.

anna v
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  • Good point about the bound system. I'm not even sure if it's possible to have "box" from whose walls photons can bounce off of. If it's possible, then maybe we can have a closed system containing photons that themselves don't interact with each other? – Shirish Kulhari Dec 28 '19 at 08:59
  • the video I lnked is a more complicated form of this type of photon trapping . if one has a bunch of photons, it will be eaten up very soon by the walls of the box. see this and the link there https://physics.stackexchange.com/questions/521519/can-we-keep-a-photon-bouncing-of-mirrors-for-a-long-time-eg-days/521558#521558 – anna v Dec 28 '19 at 09:37