Forest UCM Ch3 CoM

The Center of mass

Definition of the Center of Mass

The position [math]\vec R[/math] of the center of mass is given by

[math]\vec{R} = \frac{\sum_i^N m_i \vec{r}_i}{\sum_i^N m_i}[/math]

The center of mass is given as the sum of the position of each object in the system weighted by the objects mass.

For a rigid object the location of the center of mass is given by

[math]\vec{R} = \frac{1}{M} \int \vec{r} dm[/math]

Example 1: CM of three particles

Calculate the location of the center of mass given the three particles below

[math]\vec{r}_1 = (1,1,0) = \hat i + \hat j[/math]

[math]\vec{r}_2 = (1,-1,0) = \hat i - \hat j[/math]

[math]\vec{r}_3 = (0,0,0) = \vec{0}[/math]

when

[math]m_1 = m_2 = 3 m_3[/math]

let [math] m_3 = M[/math]

[math]\vec{R} = \frac{3M (1) + 3 M (1) + M(0)}{5M} \hat i + \frac{3M (1) + 3 M (-1) + M(0)}{5M} \hat j[/math]

[math]\vec{R} = \frac{6}{5} \hat i + \frac{0}{5M} \hat j[/math]

Example 3: CM of a semicircle

Forest_UCM_MnAM#Center_of_Mass