Centre of mass (discrete particles) (r_cm = Σ m_i r_i / M)

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r_cm = Σ m_i r_i / M

What each symbol means

SymbolWhat it stands for
MTotal mass Σ m_i (kg)
m_iMass of ith particle (kg)
r_iPosition vector from a chosen origin (m)

When to use this

Inertial frame; same origin for all r_i. Extended bodies can be sliced into particles.