Discovering the Origins of Conservation Laws in Particle Simulations

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The discussion explores how conservation laws in particle simulations emerge from fundamental principles like F=ma, action-reaction, and geometric relationships. Despite not explicitly programming conservation properties, simulations naturally exhibit conservation of linear momentum, angular momentum, and energy due to underlying symmetries. These conservation laws are linked to specific symmetries: energy conservation relates to time translation symmetry, momentum to spatial translation symmetry, and angular momentum to rotational symmetry. The implementation of these equations must maintain these symmetries for conservation to hold, although some quantities may only be conserved on average due to numerical discretization. Understanding these connections can be complex, often requiring knowledge of concepts like Noether's theorem and differentiable symmetries.
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Let's assume I simulate a number of particles using a computer program. I teach the particles to move according to F=ma. The F acting on each particle will be the sum of all forces to other particles according to F=m1*m2/distance^2.

I give the particles a set of initial positions and velocities and I automatically get conservation of linear momentum, conservation of angular momentum, and conservation of energy, just like that. I haven't actually programmed those properties into code.

Where does the conservation come from?

Basically I've only used F=m*a, v=a*t, p=v*t, action=reaction, the pythagorean theorem and some sums. Does it come from F=m*a, from action=reaction, is it a property of isometric space, or all of them?
 
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To understand the theorem, I'd need to understand what a differentiable symmetry is, what Lagrangian is, and so forth. If I knew all that, I wouldn't be asking.

Can it be pinpointed down to something more simple? Can you give a simplified answer?
 
The simple answer is that each conservation law is related to a symmetry. Conservation of energy comes from the symmetry of translation in time, conservation of momentum translation symmetry in space, and angular momentum rotation symmetry. If your equations (and importantly their numerical implementation) have these symmetries, then the quantities will be conserved.

Note that depending on the implementation (for instance, because of time discretization), some quantities might be conserved only on average.
 
Thanks!
 
I have recently been really interested in the derivation of Hamiltons Principle. On my research I found that with the term ##m \cdot \frac{d}{dt} (\frac{dr}{dt} \cdot \delta r) = 0## (1) one may derivate ##\delta \int (T - V) dt = 0## (2). The derivation itself I understood quiet good, but what I don't understand is where the equation (1) came from, because in my research it was just given and not derived from anywhere. Does anybody know where (1) comes from or why from it the...

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