Mass center problem

1. Jan 22, 2006

question:
three uniform disks of the same mass per unit area, and radii a, 2a, 3a are placed in contact with each other with their centers on a straight line. how far is the center of mass of the system from the center of the smallest disk?

solution:
m1=pi a^2
m2=4pi a^2
m3=9pi a^2

Xc = (m1/mtotal)Xo + (m2/mtotal)(Xo+3a)+(m3/mtotal)(Xo+8a)
Xc = 1/14Xo + 4/14Xo + 6/7a + 9/14Xo + 72/14a
Xc = Xo + 6a

therefore center of mass of system is 6a from the center of the smallest disk.

is this correct?

2. Jan 22, 2006

Tide

No, that is not correct. Think about what your answer means if a is made arbitrarily small. Also, it appears there is missing information in the way you state the problem. Don't you suppose the thickness of the disks matters?

3. Jan 23, 2006

i dont really understand whats wrong with it
but here is the diagram, i guess i did miss this

http://img63.imageshack.us/img63/9140/phys27rv.jpg [Broken]

Last edited by a moderator: May 2, 2017
4. Jan 23, 2006