Definition:
**r**_{CM} = ∑
m_{i} **r**_{i}/∑ m_{i} = ∑
m_{i} **r**_{i}/ M → ∫ **r** dm/ M

We rewrite this definition as:
M **r**_{CM} = ∑
m_{i} **r**_{i}

The total momentum is: **
p**_{tot} = ∑ m_{i} **v**_{i} =
M **v**_{CM}

Newton's 2nd law:
d** p**_{tot}/dt = M
**a**_{CM} = **F**_{ext}

Conclusion: A composite object is equivalent
to a single point of mass M that is located at the center of mass.