# center of mass, total momentum

Has someone an idea how to solve the 1. question for ANY number of mass points and a clue to the 2. question?

center of mass system, momentum

• Didn't you tell me to do it like this in main question?? – Tom Jan 14 '19 at 9:24
• Yes, but I didn't mean something like the above which just references the previous question. I meant a fully written out question with the full problem statement so the new question can stand alone. – m_goldberg Jan 14 '19 at 9:34
• @Tom Indeed, a bit more self-containedness of this question would be desirable. When I read it the first time I thought "What the ... do they mean with '1. question'?" – Henrik Schumacher Jan 14 '19 at 9:38
• ok, in future I will try to do it more correctly! thank you for patience ! – Tom Jan 14 '19 at 9:42
• Near duplicate: mathematica.stackexchange.com/questions/189362/… – David G. Stork Jan 14 '19 at 18:43

The data for many particles are best stored in arrays.

n = 10000;
masses = RandomReal[{0.1, 1.}, {n}];
positions = RandomReal[{-1, 1}, {n, 3}];
velocities = RandomReal[{-1, 1}, {n, 3}];


Now, one obtains

centerofmass = masses.positions;
momenta = masses velocities;
totalmomentum = masses.velocities;
totalangularmomentum = Total[angularmomenta];


# Performance Tuning

There is an issue with Cross: For some reason, it is way slower than it should be. Here is a compiled, listable, and parallelized version of it for 3-vectors:

cross = With[{
code = Cross[
Table[CompileGetElement[X, i], {i, 1, 3}],
Table[CompileGetElement[Y, i], {i, 1, 3}]
]
},
Compile[{{X, _Real, 1}, {Y, _Real, 1}},
code,
CompilationTarget -> "C",
RuntimeAttributes -> {Listable},
Parallelization -> True,
RuntimeOptions -> "Speed"
]
]


It is about 500 times faster than Cross on my machine and it is already Listable so that we don't need MapThread:

angularmomenta2 = cross[positions, momenta];


Up to machine precision, the result is the same:

Max[Abs[angularmomenta - angularmomenta2]]


2.22045*10^-16

• you´re genious Henrik! thank you! – Tom Jan 14 '19 at 9:43
• Huh, thanks for the flowers. You're welcome. – Henrik Schumacher Jan 14 '19 at 9:43