# Center of gravity

1. Dec 11, 2005

### Greenrose

i can get the answer i should for a ball with radius 0.0800m and mass 1.00kg is attached to a rod 0.400m long in length to a second ball with radius 0.100m and mass 2.00kg. Where is the center of gravity of the system.
i used the formula Xcg = (M*R + m*r)/(M+m) and get an answer of 0.093 but the answer in the back of the book says i should get 0.387m from 1kg sphere. Am i using the correct formula?

2. Dec 11, 2005

### Staff: Mentor

The formula for center of mass should be:
$$x_{CM} = (x_1 m_1 + x_2 m_2)/(m_1 + m_2)$$

What are the coordinates of each ball measured from the center of the 1 kg ball?

3. Dec 11, 2005

### dicerandom

If they want the distance from the 1kg sphere you should use the center of that sphere as the origin of your coordinate system. The equation you gave is correct, think carefully about what your values for R and r should be.

4. Dec 11, 2005

### Greenrose

There are no coordinates of each ball measured from the center of the 1 kg ball, and im not sure what differerent values of r to use i just used
(0.0800*1.00 + 0.100*2.00)/(2.00+1.00) any help would be appreciated

5. Dec 11, 2005

### Staff: Mentor

You have to figure them out! Draw a diagram.

Note: Assuming that R and r are the radii of the two balls, the equation you used is incorrect. (You will need the radii to figure out the coordinates though.)

6. Dec 11, 2005

### Greenrose

thanks very much i understand now