\magnification=\magstep2


$$
X_{com} = { {x_1m_1 + x_2m_2 + ... + x_Nm_N} \over
             { m_1 + m_2 + ... + m_N } }
$$

$$
Y_{com} = { {y_1m_1 + y_2m_2 + ... + y_Nm_N} \over
             { m_1 + m_2 + ... + m_N } }
$$



$$
X_{com} = { {\sum x_i m_i} \over
             { \sum m_i} }
$$

$$
Y_{com} = { {\sum y_i m_i} \over
             { \sum m_i} }
$$

$$
Z_{com} = { {\sum z_i m_i} \over
             { \sum m_i} }
$$


$$
X_{com} = { {\sum x_i m_i} \over { M } }
$$

$$
Y_{com} = { {\sum y_i m_i} \over { M } }
$$

$$
Z_{com} = { {\sum z_i m_i} \over { M } }
$$

\vfill\eject

$$
\left( M X_{com}, M Y_{com}, M Z_{com} \right)  = 
           { \left( \sum x_i m_i , \sum y_i m_i , \sum z_i m_i \right) }
$$

$$
 M V_{x}  = { \sum v_i m_i  }
$$


$$
 \vec{p}  = { m \vec{v}  }
$$

$$
 {{\vec{p}}_{tot}}  = \sum{\vec{p_i} }  = { \sum { m_i \vec{v_i}  } }
$$





\bye
