Center of Gravity: Finding 0.387m from 1kg Sphere?”

[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?

 

[Doc Al]

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?

 

[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.

 

[Greenrose]

There are no coordinates of each ball measured from the center of the 1 kg ball, and I am 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

 

[Doc Al]

There are no coordinates of each ball measured from the center of the 1 kg ball,

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.)

 

[Greenrose]

thanks very much i understand now